show ip interface

To get a detailed listing of all the IP-related characteristics of an interface, use the show ip interface command. A common use for this command is to view any secondary addresses that have been assigned to an interface (they do not show up in the standard show interface output).

show ip interface [brief]

Example

Router# show ip interface

Ethernet0 is administratively down, line protocol is down
  Internet address is 10.10.46.10, subnet mask is 255.0.0.0
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is enabled
  Multicast groups joined: 224.0.0.1 224.0.0.2
  Outgoing access list is not set
  Inbound access list is not set
  Proxy ARP is enabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP fast switching on the same interface is disabled
  IP SSE switching is disabled
  Router Discovery is disabled
  IP accounting is disabled
  TCP/IP header compression is disabled
  Probe proxy name replies are disabled
  Gateway Discovery is disabled
  

Looking at the output section by section:

Ethernet0 is administratively down, line protocol is down

This line is same as the normal show interface output. It indicates the current status of the interface and configured protocol.

Internet address is 10.10.46.10, subnet mask is 255.0.0.0
Broadcast address is 255.255.255.255
Address determined by setup command

These lines give IP address, subnet mask, and broadcast address that the interface is listening for and how that information was attained.

MTU is 1500 bytes

This is the maximum size of a packet going through the interface.

Helper address is not set

This line indicates if an IP helper address has been configured. IP Helper addresses are used when the destination network address for an IP packet is not known, such as for a workstation using the BOOTP protocol.

Directed broadcast forwarding is enabled

This tells you that the interface will forward a broadcast intended for the subnet configured on the interface.

Multicast groups joined: 224.0.0.1 224.0.0.2

This line indicates that the interface has joined the specified multicast groups, the default.

Outgoing access list is not set
Inbound access list is not set
  

No access lists are being applied to either inbound or outbound datagrams while passing through this interface.

Proxy ARP is enabled

This output indicates that the router will respond to an ARP request not destined for its own MAC address if it knows the network that the ARP request is looking for.

Split horizon is enabled

The interface is following the standard split horizon rule. It will not forward a route it learned from this interface back out this interface.

ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
  

The interface will handle Internet Control Message Protocol (ICMP) messages in the stated manner.

IP fast switching is enabled
IP fast switching on the same interface is disabled
IP SSE switching is disabled
  

These lines indicate how the router is switching packets. Of particular use is noting if fast switching is enabled or not. In some debuging modes, packets will not be able to be analyzed if they are being fast switched.

Router Discovery is disabled

The router discovery protocol Interdomain Policy Routing, or IDRP, is not enabled for this interface.

IP accounting is disabled

IP accounting is not enabled for this interface.

TCP/IP header compression is disabled

TCP/IP header compression is not enabled for the interface.

You can also use the show ip interface command with the optional argument brief to get a quick list of what primary IP addresses have been assigned to all the interfaces in the router, as well as the status of the interface.

Router# show ip interface brief

Interface              IP-Address     OK? Method Status                Protocol
Ethernet0              172.16.151.56  YES NVRAM  up                    up
Serial0                130.10.1.5     YES NVRAM  administratively down down
Serial1                unassigned     YES unset  administratively down down
  

In the sample output above, notice that two interfaces have assigned IP addresses, but only Ethernet 0 is up/up. Also indicated is the method for each interface getting its address, in this case, nonvolatile RAM (NVRAM) (that is, the configuration file).