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DNS - VRF Aware DNS

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VRF-Aware DNS

Table Of Contents

VRF-Aware DNS

Contents

Information About VRF-Aware DNS

Domain Name System

VRF Mapping and VRF-Aware DNS

How to Configure VRF-Aware DNS

Defining a VRF Table and Assigning a Name Server to Enable VRF-Aware DNS

Mapping VRF-Specific Hostnames to IP Addresses

Configuring a Static Entry in a VRF-Specific Name Cache

Verifying the Name Cache Entries in the VRF Table

Configuration Examples for VRF-Aware DNS

VRF-Specific Name Server Configuration: Example

VRF-Specific Domain Name List Configuration: Example

VRF-Specific Domain Name Configuration: Example

VRF-Specific IP Host Configuration: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

clear host

ip domain list

ip domain name

ip host

ip name-server

show hosts


VRF-Aware DNS


The VRF-Aware DNS feature enables the configuration of a Virtual Private Network (VPN) routing and forwarding instance (VRF) table so that the domain name system (DNS) can forward queries to name servers using the VRF table rather than the named DNS server in the global IP address space. This feature allows DNS requests to be resolved within the appropriate Multiprotocol Label Switching (MPLS) VPN.

History for the VRF-Aware DNS Feature

Release
Modification

12.4(4)T

This feature was introduced.


Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Contents

Information About VRF-Aware DNS

How to Configure VRF-Aware DNS

Configuration Examples for VRF-Aware DNS

Additional References

Command Reference

Information About VRF-Aware DNS

To configure the VRF-Aware DNS feature, you should understand the following concepts:

Domain Name System

VRF Mapping and VRF-Aware DNS

Domain Name System

Domain Name System (DNS) is a standard that defines a domain naming procedure used in TCP/IP. A domain is a hierarchical separation of the network into groups and subgroups with domain names identifying the structure. The named groups consist of named objects, usually devices like IP hosts, and the subgroups are domains. DNS has three basic functions:

Name space: This function is a hierarchical space organized from a single root into domains. Each domain can contain device names or more specific information. A special syntax defines valid names and identifies the domain names.

Name registration: This function is used to enter names into the DNS database. Policies are outlined to resolve conflicts and other issues.

Name resolution: This function is a distributed client and server name resolution standard. The name servers are software applications that run on a server and contain the resource records (RRs) that describe the names and addresses of those entities in the DNS name space. A name resolver is the interface between the client and the server. The name resolver requests information from the server about a name. A cache can be used by the name resolver to store learned names and addresses.

A DNS server can be a dedicated device or a software process running on a device. The server stores and manages data about domains and responds to requests for name conflict resolutions. In a large DNS implementation, there can be a distributed database over many devices. A server can be a dedicated cache.

VRF Mapping and VRF-Aware DNS

To keep track of domain names, IP has defined the concept of a name server, whose job is to hold a cache (or database) of names appended to IP addresses. The cached information is important because the requesting DNS will not need to query for that information again, which is why DNS works well. If a server had to query each time for the same address because it had not saved any data, the queried servers would be flooded and would crash.

A gateway for multiple enterprise customers can be secured by mapping the remote users to a VRF domain. Mapping means obtaining the IP address of the VRF domain for the remote users. By using VRF domain mapping, a remote user can be authenticated by a VRF domain-specific AAA server so that the remote-access traffic can be forwarded within the VRF domain to the servers on the corporate network.

To support traffic for multiple VRF domains, the DNS and the servers used to resolve conflicts must be VRF aware. VRF aware means that a DNS subsystem will query the VRF name cache first, then the VRF domain, and store the returned RRs in a specific VRF name cache. Users are able to configure separate DNS name servers per VRF.

VRF-aware DNS forwards queries to name servers using the VRF table. Because the same IP address can be associated with different DNS servers in different VRF domains, a separate list of name caches for each VRF is maintained. The DNS looks up the specific VRF name cache first, if a table has been specified, before sending a query to the VRF name server. All IP addresses obtained from a VRF-specific name cache are routed using the VRF table.

How to Configure VRF-Aware DNS

This section contains the following tasks:

Defining a VRF Table and Assigning a Name Server to Enable VRF-Aware DNS

Mapping VRF-Specific Hostnames to IP Addresses

Configuring a Static Entry in a VRF-Specific Name Cache

Verifying the Name Cache Entries in the VRF Table

Defining a VRF Table and Assigning a Name Server to Enable VRF-Aware DNS

Perform this task to define a VRF table and assign a name server.

A VRF-specific name cache is dynamically created if one does not exist whenever a VRF-specific name server is configured by using the ip name-server vrf command option or a permanent name entry is configured by using the ip host vrf command option. The VRF name cache is removed whenever all name server and permanent entries in the VRF are disabled.

It is possible that multiple name servers are configured with the same VRF name. The system will send queries to those servers in turn until any of them responds, starting with the server that sent a response the last time.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip vrf vrf-name

4. rd route-distinguisher

5. exit

6. ip name-server [vrf vrf-name] server-address1 [server-address2...server-address6]

7. ip domain lookup

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

ip vrf vrf-name

Example:

Router(config)# ip vrf vpn1

Defines a VRF table and enters VRF configuration mode.

The vrf-name argument can be up to 32 characters.

Step 4 

rd route-distinguisher

Example:

Router(config)# rd 100:21

Creates routing and forwarding tables for a VRF.

Step 5 

exit

Example:

Router(config-vrf) exit

Exits VRF configuration mode.

Step 6 

ip name-server [vrf vrf-name] server-address1 [server-address2...server-address6]

Example:

Router(config)# ip name-server vrf vpn1 172.16.1.111 172.16.1.2

Assigns the address of one or more name servers to a VRF table to use for name and address resolution.

The vrf keyword is optional but must be specified if the name server is used with VRF. The vrf-name argument assigns a name to the VRF.

Step 7 

ip domain lookup

Example:

Router(config)# ip domain lookup

(Optional) Enables DNS-based address translation.

DNS is enabled by default. You only need to use this command if DNS has been disabled.

Mapping VRF-Specific Hostnames to IP Addresses

Perform this task to map VRF-specific hostnames to IP addresses.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip domain name [vrf vrf-name] name
or
ip domain list [vrf vrf-name] name

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

ip domain name [vrf vrf-name] name

or

ip domain list [vrf vrf-name] name

Example:

Router(config)# ip domain name vrf vpn1 cisco.com

or

Example:

Router(config)# ip domain list vrf vpn1 cisco.com

Defines a default domain name that the Cisco IOS software will use to complete unqualified hostnames.

or

Defines a list of default domain names to complete unqualified hostnames.

You can specify a default domain name that the Cisco IOS software will use to complete domain name requests. You can specify either a single domain name or a list of domain names. Any hostname that does not contain a complete domain name will have the default domain name you specify appended to it before the name is looked up.

The vrf keyword and vrf-name argument specify a default VRF domain name.

The ip domain list command can be entered multiple times to specify more than one domain name to append when doing a DNS query. The system will append each in turn until it finds a match.

Configuring a Static Entry in a VRF-Specific Name Cache

Perform this task to configure a static entry in a VRF-specific name cache.

A VRF-specific name cache is dynamically created if one does not exist whenever a name server is configured for the VRF by using the ip name-server vrf command option or a permanent name entry is configured by using the ip host vrf command option. The VRF name cache is removed whenever all name server and permanent entries in the VRF are disabled.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip host [vrf vrf-name] name [tcp-port-number] address1 [address2...address8]

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

ip host [vrf vrf-name] name [tcp-port] address1 [address2...address8]


Example:

Router(config)# ip host vrf vpn3 company1.com 172.16.2.1

Defines a static hostname-to-address mapping in the host cache.

If the vrf keyword and vrf-name arguments are specified, then a permanent entry is created only in the VRF-specific name cache.

Verifying the Name Cache Entries in the VRF Table

Perform this task to verify the name cache entries in the VRF table.

SUMMARY STEPS

1. enable

2. show hosts [vrf vrf-name] {all | hostname} [summary]

3. clear host [vrf vrf-name] {all | hostname}

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

show hosts [vrf vrf-name] {all | hostname} [summary]

Example:

Router# show hosts vrf vpn2

Displays the default domain name, the style of name lookup service, a list of name server hosts, the cached list of hostnames and addresses, and the cached list of hostnames and addresses specific to a particular Virtual Private Network (VPN).

The vrf keyword and vrf-name argument only display the entries if a VRF name has been configured.

If you enter the show hosts command without specifying any VRF, only the entries in the global name cache will display.

Step 3 

clear host [vrf vrf-name] {all | hostname}

Example:

Router# clear host vrf vpn2

(Optional) Deletes entries from the hostname-to-address global address cache or VRF name cache.

Configuration Examples for VRF-Aware DNS

This section provides the following configuration examples:

VRF-Specific Name Server Configuration: Example

VRF-Specific Domain Name List Configuration: Example

VRF-Specific Domain Name Configuration: Example

VRF-Specific IP Host Configuration: Example

VRF-Specific Name Server Configuration: Example

The following example shows how to specify a VPN named vpn1 with the IP addresses of 172.16.1.111 and 172.16.1.2 as the name servers:

ip name-server vrf vpn1 172.16.1.111 172.16.1.2 

VRF-Specific Domain Name List Configuration: Example

The following example shows how to add several domain names to a list in vpn1 and vpn2. The domain name is only used for name queries in the specified VRF.

ip domain list vrf vpn1 company.com
ip domain list vrf vpn2 school.edu

If there is no domain list, the domain name that you specified with the ip domain name global configuration command is used. If there is a domain list, the default domain name is not used. The ip domain list command is similar to the ip domain name command, except that with the ip domain list command you can define a list of domains, each to be tried in turn until a match is found.

VRF-Specific Domain Name Configuration: Example

The following example shows how to define cisco.com as the default domain name for a VPN named vpn1. The domain name is only used for name queries in the specified VRF.

ip domain name vrf vpn1 cisco.com

Any IP hostname that does not contain a domain name (that is, any name without a dot) will have the dot and cisco.com appended to it before being looked up.

VRF-Specific IP Host Configuration: Example

The following example shows how to define two static hostname-to-address mappings in the host cache for vpn2 and vpn3:

ip host vrf vpn2 host2 10.168.7.18
ip host vrf vpn3 host3 10.12.0.2 

Additional References

The following sections provide references related to the VRF-Aware DNS feature.

Related Documents

Related Topic
Document Title

IP addressing services configuration tasks

Cisco IOS IP Addressing Services Configuration Guide, Release 12.4

IP addressing services commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples

Cisco IOS IP Addressing Services Command Reference, Release 12.4T


Standards

Standards
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.


MIBs

MIBs
MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFCs
Title

No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.


Technical Assistance

Description
Link

The Cisco Technical Support website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

http://www.cisco.com/techsupport


Command Reference

This section documents modified commands only.

clear host

ip domain list

ip domain name

ip host

ip name-server

show hosts

clear host

To delete entries from the hostname-to-address cache, use the clear host command in EXEC mode.

clear host [vrf vrf-name] {all | hostname}

Syntax Description

vrf vrf-name

(Optional) Virtual Private Network (VPN) routing and forwarding (VRF).

all

Removes all entries.

hostname

Particular hostname to remove.


Command Modes

EXEC

Command History

Release
Modification

10.0

This command was introduced.

12.4(4)T

The vrf keyword, vrf-name argument, and all keyword were added.


Usage Guidelines

The host name entries will not be removed from NVRAM, but will be cleared in running memory.

Examples

The following example shows how to clear all entries from the hostname-to-address cache:

Router> clear host all

The following example shows how to clear all entries from the hostname-to-address cache in the VPN name vpn1:

Router> clear host vrf vpn1

Related Commands

Command
Description

ip host

Defines a static host name-to-address mapping in the host cache.

show hosts

Displays the default domain name, the style of name lookup service, a list of name server hosts, and the cached list of host names and addresses.


ip domain list

To define a list of default domain names to complete unqualified names, use the ip domain list command in global configuration mode. To delete a name from a list, use the no form of this command.

ip domain list [vrf vrf-name] name

no ip domain list [vrf vrf-name] name

Syntax Description

vrf vrf-name

(Optional) Defines a Virtual Private Network (VPN) routing and forwarding instance (VRF) table. The vrf-name argument specifies a name for the VRF table.

name

Domain name. Do not include the initial period that separates an unqualified name from the domain name.


Defaults

No domain names are defined.

Command Modes

Global configuration

Command History

Release
Modification

10.0

This command was introduced.

12.2

The syntax of the command changed from ip domain-list to ip domain list.

12.4(4)T

The vrf keyword and vrf-name argument were added.


Usage Guidelines

If there is no domain list, the domain name that you specified with the ip domain name global configuration command is used. If there is a domain list, the default domain name is not used. The ip domain list command is similar to the ip domain name command, except that with the ip domain list command you can define a list of domains, each to be tried in turn until the system finds a match.

If the ip domain list vrf command option is specified, the domain names are only used for name queries in the specified VRF.

The Cisco IOS software will still accept the previous version of the command, ip domain-list.

Examples

The following example shows how to add several domain names to a list:

ip domain list company.com
ip domain list school.edu 

The following example shows how to add several domain names to a list in vpn1 and vpn2:

ip domain list vrf vpn1 company.com
ip domain list vrf vpn2 school.edu 


Related Commands

Command
Description

ip domain list

Defines a list of default domain names to complete unqualified hostnames.

ip domain lookup

Enables the IP DNS-based hostname-to-address translation.

ip domain retry

Specifies the number of times to retry sending DNS queries.

ip domain timeout

Specifies the amount of time to wait for a response to a DNS query.

ip name-server

Specifies the address of one or more name servers to use for name and address resolution.


ip domain name

To define a default domain name that the Cisco IOS software uses to complete unqualified hostnames (names without a dotted-decimal domain name), use the ip domain name command in global configuration mode. To disable use of the Domain Name System (DNS), use the no form of this command.

ip domain name [vrf vrf-name] name

no ip domain name [vrf vrf-name] name

Syntax Description

vrf vrf-name

(Optional) Defines a Virtual Private Network (VPN) routing and forwarding instance (VRF) table. The vrf-name argument specifies a name for the VRF table.

name

Default domain name used to complete unqualified hostnames. Do not include the initial period that separates an unqualified name from the domain name.


Defaults

Enabled

Command Modes

Global configuration

Command History

Release
Modification

10.0

This command was introduced.

12.2

The syntax of the command changed from ip domain-name to ip domain name.

12.4(4)T

The vrf keyword and vrf-name argument were added.


Usage Guidelines

Any IP hostname that does not contain a domain name (that is, any name without a dot) will have the dot and cisco.com appended to it before being added to the host table.

If the ip domain name vrf command option is specified, the domain names are only used for name queries in the specified VRF.

The Cisco IOS software will still accept the previous version of the command, which is ip domain-name.

Examples

The following example shows how to define cisco.com as the default domain name:

ip domain name cisco.com

The following example shows how to define cisco.com as the default domain name for vpn1:

ip domain name vrf vpn1 cisco.com

Related Commands

Command
Description

ip domain list

Defines a list of default domain names to complete unqualified hostnames.

ip domain lookup

Enables the IP DNS-based hostname-to-address translation.

ip domain retry

Specifies the number of times to retry sending DNS queries.

ip domain timeout

Specifies the amount of time to wait for a response to a DNS query.

ip name-server

Specifies the address of one or more name servers to use for name and address resolution.


ip host

To define a static hostname-to-address mapping in the host cache, use the ip host command in global configuration mode. To remove the hostname-to-address mapping, use the no form of this command.

ip host [vrf vrf-name] {name | tmodem-telephone-number} [tcp-port-number] address1 [address2...address8]

no ip host [vrf vrf-name] {name | tmodem-telephone-number} [tcp-port-number] address1 [address2...address8]

Syntax Description

vrf vrf-name

(Optional) Defines a Virtual Private Network (VPN) routing and forwarding instance (VRF) table. The vrf-name argument specifies a name for the VRF table.

name

Name of the host. The first character can be either a letter or a number. If you use a number, the types of operations you can perform are limited.

tmodem-telephone-number

Modem telephone number that is mapped to the IP host address for use in Cisco modem user interface mode. You must enter the letter "t" before the telephone number.

tcp-port-number

(Optional) TCP port number to connect to when using the defined hostname in conjunction with an EXEC connect or Telnet command. The default is Telnet (port 23).

address1

Associated IP host address.

address2...address8

(Optional) Additional associated IP addresses. You can bind up to eight addresses to a hostname.


Defaults

Disabled

Command Modes

Global configuration

Command History

Release
Modification

10.0

This command was introduced.

12.2(4)T

The capability to map a modem telephone number to an IP host was added for the Cisco modem user interface feature.

12.4(4)T

The vrf keyword and vrf-name argument were added.


Usage Guidelines

The first character can be either a letter or a number. If you use a number, the types of operations you can perform (such as ping) are limited.

Examples

The following example shows how to define two static mappings:

ip host host2 10.168.7.18
ip host host3 10.2.0.2 192.168.7.33

The following example shows how to define two static hostname-to-address mappings in the host cache for vpn2 and vpn3:

ip host vrf vpn2 host2 172.16.3.1
ip host vrf vpn3 host3 172.18.4.1

The following example shows how to map modem telephone number 555-1234 to IP host address 10.1.5.5 for the Cisco modem user interface mode:

ip host t5551234 10.1.5.5

ip name-server

To specify the address of one or more name servers to use for name and address resolution, use the ip name-server command in global configuration mode. To remove the addresses specified, use the no form of this command.

ip name-server [vrf vrf-name] server-address1 [server-address2...server-address6]

no ip name-server [vrf vrf-name] server-address1 [server-address2...server-address6]

Syntax Description

vrf vrf-name

(Optional) Defines a Virtual Private Network (VPN) routing and forwarding instance (VRF) table. The vrf-name argument specifies a name for the VRF table.

server-address1

IPv4 or IPv6 addresses of a name server.

server-address2...server-address6

(Optional) IP addresses of additional name servers (a maximum of six name servers).


Defaults

No name server addresses are specified.

Command Modes

Global configuration

Command History

Release
Modification

10.0

This command was introduced.

12.2(2)T

Support for IPv6 addresses was added.

12.0(21)ST

Support for IPv6 addresses was added.

12.0(22)S

Support for IPv6 addresses was added.

12.2(14)S

Support for IPv6 addresses was added.

12.2(27)SBC

This command was integrated into Cisco IOS Release 12.2(27)SBC.

12.4(4)T

The vrf keyword and vrf-name argument were added.


Examples

The following example shows how to specify IPv4 hosts 172.16.1.111 and 172.16.1.2 as the name servers:

ip name-server 172.16.1.111 172.16.1.2

This command will be reflected in the configuration file as follows:

ip name-server 172.16.1.111
ip name-server 172.16.1.2

The following example shows how to specify IPv4 hosts 172.16.1.111 and 172.16.1.2 as the name servers for vpn1:

Router(config)# ip name-server vrf vpn1 172.16.1.111 172.16.1.2


The following example shows how to specify IPv6 hosts 3FFE:C00::250:8BFF:FEE8:F800 and 2001:0DB8::3 as the name servers:

ip name-server 3FFE:C00::250:8BFF:FEE8:F800 2001:0DB8::3 

This command will be reflected in the configuration file as follows:

ip name-server 3FFE:C00::250:8BFF:FEE8:F800 
ip name-server 2001:0DB8::3

Related Commands

Command
Description

ip domain-lookup

Enables the IP DNS-based hostname-to-address translation.

ip domain-name

Defines a default domain name to complete unqualified hostnames (names without a dotted decimal domain name).


show hosts

To display the default domain name, the style of name lookup service, a list of name server hosts, the cached list of hostnames and addresses, and the cached list of hostnames and addresses specific to a particular Virtual Private Network (VPN), use the show hosts command in EXEC mode.

show hosts [vrf vrf-name] {all | hostname} [summary]

Syntax Description

vrf vrf-name

(Optional) Defines a Virtual Private Network (VPN) routing and forwarding instance (VRF) table. The vrf-name argument specifies a name for the VRF table. Only the name cache entries for a particular VRF will be displayed.

all

Displays all name cache entries.

hostname

Displays the name cache entries for a particular hostname.

summary

(Optional) Displays a brief summary of the name caches.


Command Modes

EXEC

Command History

Release
Modification

10.0

This command was introduced.

12.2(4)T

This command was updated to support the Cisco modem user interface feature.

12.4(4)T

The vrf, all, and summary keywords and vrf-name and hostname arguments were added.


Usage Guidelines

If you specify the show hosts command without the optional keywords, only the entries in the global name cache will be displayed.

Examples

The following is sample output from the show hosts command:

Router# show hosts

Default domain is CISCO.COM
Name/address lookup uses domain service
Name servers are 255.255.255.255
Host	              Flag        Age	   Type	       Address(es)
SLAG.CISCO.COM    (temp, OK)  1     IP         172.20.4.10
CHAR.CISCO.COM    (temp, OK)  8     IP         192.168.7.50
CHAOS.CISCO.COM   (temp, OK)  8     IP         172.20.1.115
DIRT.CISCO.COM    (temp, EX)  8     IP         172.20.1.111
DUSTBIN.CISCO.COM (temp, EX)  0     IP         172.20.1.27
DREGS.CISCO.COM   (temp, EX)  24    IP         172.20.1.30

The following is sample output from the show hosts vrf command with a VRF name red specified:

Router# show hosts vrf red 

Name lookup view: red 
Default domain is red.com 
Domain list: red1.com, red2.com 
Name/address lookup uses domain service 
Name servers are 10.0.0.2, 10.0.0.3, 10.0.0.4 

Codes: UN - unknown, EX - expired, OK - OK,  
        temp - temporary, perm - permanent  
         NA - Not Applicable None - Not defined 

Host                     Port  Flags    Age   Type   Address(es)  
a.red.com                None (temp, OK) 0     IP     10.0.0.2 
printer.red.com          None (perm, OK) 0     IP     10.0.0.5 
                                                           10.0.0.7
dns.red.com                None (perm, OK)   0      IP     10.0.0.2 

Table 1 describes the significant fields shown in the display.

Table 1 show hosts Field Descriptions 

Field
Description

Flag

A temporary entry is entered by a name server; the Cisco IOS software removes the entry after 72 hours of inactivity.

A permanent entry is entered by a configuration command and is not timed out. Entries marked OK are believed to be valid. Entries marked ?? are considered suspect and subject to revalidation. Entries marked EX are expired.

Age

Indicates the number of hours since the software last referred to the cache entry.

Type

Identifies the type of address, for example, IP, Connectionless Network Service (CLNS), or X.121. If you have used the ip hp-host global configuration command, the show hosts command will display these hostnames as type HP-IP.

Address(es)

Displays the address of the host. One host may have up to eight addresses.


The following is sample output from a router when a modem telephone number is mapped to an IP host address for the Cisco modem user interface feature using the ip host global configuration command:

Router# show hosts

Default domain is not set
Name/address lookup uses domain service
Name servers are 255.255.255.255

Codes: u - unknown, e - expired, * - OK, ? - revalidate
       t - temporary, p - permanent

   Host                   Age  Type     Address(es)
*p p4085554567             0   IP        10.2.1.6
*p t4085551234             0   IP        10.2.1.5

Under the Host field, a "p" preceding the number indicates a pulse-dialed modem telephone number, and a "t" indicates a tone-dialed modem telephone number. The IP address mapped to the telephone number appears under the Address(es) field. See Table 1 for descriptions of the other fields seen in this display.

Related Commands

Command
Description

clear arp interface

Deletes entries from the hostname-to-address cache.

ip helper-address

Defines a static host-name-to-address mapping in the host cache.