IP Addressing and Subnetting for New Users
Contenido
Introducción
Este documento proporciona la información básica necesaria para configurar el router para el ruteo de IP, como por ejemplo, cómo se descomponen las direcciones y cómo funciona la conexión en subredes. You learn how to assign each interface on the router an IP address with a unique subnet. Se incluyen ejemplos para ayudar a comprender todo.
Prerequisites
Requirements
Cisco recomienda tener un conocimiento básico de los números binarios y decimales.
Componentes Utilizados
Este documento no tiene restricciones específicas en cuanto a versiones de software y de hardware.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Additional Information
Si las definiciones le son de ayuda, utilice estos términos para comenzar:
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Dirección: el número de identificación exclusivo asignado a un host o a una interfaz en una red.
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Subred: una parte de una red que comparte una dirección de subred en particular.
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Máscara de subred: una combinación de 32 bits utilizada para describir la parte de una dirección que se refiere a la subred y la parte que se refiere a un host.
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Interfaz: una conexión de red.
If you have already received your legitimate address(es) from the Internet Network Information Center (InterNIC), you are ready to begin. Si no planea conectar con Internet, Cisco sugiere encarecidamente que utilice direcciones reservadas de RFC1918.
Explicación de las direcciones IP
Una dirección IP es una dirección empleada para identificar a un dispositivo en una red IP. La dirección se compone de 32 bits binarios, que pueden dividirse en una porción correspondiente a la red y otra correspondiente al host, con la ayuda de una máscara de subred. The 32 binary bits are broken into four octets (1 octet = 8 bits). Each octet is converted to decimal and separated by a period (dot). For this reason, an IP address is said to be expressed in dotted decimal format (for example, 172.16.81.100). The value in each octet ranges from 0 to 255 decimal, or 00000000 - 11111111 binary.
Here is how binary octets convert to decimal: El bit del extremo derecho, o el bit menos significativo, de un octeto tiene un valor de 20. El bit justo a la izquierda de ese tiene un valor de 21. Este proceso continúa así hasta el bit más a la izquierda, o bit más importante, que tiene un valor de 27. So if all binary bits are a one, the decimal equivalent would be 255 as shown here:
1 1 1 1 1 1 1 1 128 64 32 16 8 4 2 1 (128+64+32+16+8+4+2+1=255)
Here is a sample octet conversion when not all of the bits are set to 1.
0 1 0 0 0 0 0 1 0 64 0 0 0 0 0 1 (0+64+0+0+0+0+0+1=65)
Y en este ejemplo se ve una dirección IP tanto en números binarios como decimales.
10. 1. 23. 19 (decimal) 00001010.00000001.00010111.00010011 (binary)
These octets are broken down to provide an addressing scheme that can accommodate large and small networks. There are five different classes of networks, A to E. Este documento se centra en las clases A a C, puesto que las clases D y E están reservadas y no son relevantes para la finalidad de este documento.
La clase de una dirección IP se puede determinar a partir de los tres bits de gran importancia (los tres bits del extremo izquierdo en el primer octeto). Figure 1 shows the significance in the three high order bits and the range of addresses that fall into each class. For informational purposes, Class D and Class E addresses are also shown.
Figure 1
En una dirección de Clase A, el primer octeto es la parte de la red, por lo que el ejemplo de Clase A de la Figura 1 tiene una dirección de red principal de 1.0.0.0 - 127.255.255. Los octetos 2, 3 y 4 (los 24 bits siguientes) son para que el administrador de red se divida en subredes y hosts como él/ella ve fit. Class A addresses are used for networks that have more than 65,536 hosts (actually, up to 16777214 hosts!).
En una dirección de Clase B, los dos primeros octetos son la parte de la red, por lo que el ejemplo de Clase B de la Figura 1 tiene una dirección de red principal de 128.0.0.0 - 191.255.255. Los octetos 3 y 4 (16 bits) son para subredes y hosts locales. Class B addresses are used for networks that have between 256 and 65534 hosts.
In a Class C address, the first three octets are the network portion. El ejemplo de Clase C de la Figura 1 tiene una dirección de red principal de 192.0.0.0 - 223.255.255.255. El octeto 4 (8 bits) es para subredes y hosts locales, perfecto para redes con menos de 254 hosts.
Network Masks
A network mask helps you know which portion of the address identifies the network and which portion of the address identifies the node. Class A, B, and C networks have default masks, also known as natural masks, as shown here:
Class A: 255.0.0.0 Class B: 255.255.0.0 Class C: 255.255.255.0
An IP address on a Class A network that has not been subnetted would have an address/mask pair similar to: 8.20.15.1 255.0.0.0. Para ver cómo la máscara ayuda a identificar en la dirección las partes de la red y del nodo, pase la dirección y la máscara a números binarios.
8.20.15.1 = 00001000.00010100.00001111.00000001 255.0.0.0 = 11111111.00000000.00000000.00000000
Una vez que tiene la dirección y la máscara representadas en binario, la identificación de la red y el ID de host es más fácil. Any address bits which have corresponding mask bits set to 1 represent the network ID. Any address bits that have corresponding mask bits set to 0 represent the node ID.
8.20.15.1 = 00001000.00010100.00001111.00000001
255.0.0.0 = 11111111.00000000.00000000.00000000
-----------------------------------
net id | host id
netid = 00001000 = 8
hostid = 00010100.00001111.00000001 = 20.15.1
Explicación de las subredes
Subnetting allows you to create multiple logical networks that exist within a single Class A, B, or C network. If you do not subnet, you are only able to use one network from your Class A, B, or C network, which is unrealistic.
Each data link on a network must have a unique network ID, with every node on that link being a member of the same network. If you break a major network (Class A, B, or C) into smaller subnetworks, it allows you to create a network of interconnecting subnetworks. Each data link on this network would then have a unique network/subnetwork ID. Cualquier dispositivo, o gateway, que conecte n redes/subredes tiene n direcciones IP distintas, una para cada red/subred que interconecte.
Para crear una subred en una red, extienda la máscara natural con algunos de los bits de la parte del ID de host de la dirección para crear un ID de subred. Por ejemplo, dada una red de Clase C de 204.17.5.0 que tiene una máscara natural de 255.255.255.0, puede crear subredes de esta manera:
204.17.5.0 - 11001100.00010001.00000101.00000000
255.255.255.224 - 11111111.11111111.11111111.11100000
--------------------------|sub|----
By extending the mask to be 255.255.255.224, you have taken three bits (indicated by "sub") from the original host portion of the address and used them to make subnets. With these three bits, it is possible to create eight subnets. With the remaining five host ID bits, each subnet can have up to 32 host addresses, 30 of which can actually be assigned to a device since host ids of all zeros or all ones are not allowed (it is very important to remember this). So, with this in mind, these subnets have been created.
204.17.5.0 255.255.255.224 host address range 1 to 30 204.17.5.32 255.255.255.224 host address range 33 to 62 204.17.5.64 255.255.255.224 host address range 65 to 94 204.17.5.96 255.255.255.224 host address range 97 to 126 204.17.5.128 255.255.255.224 host address range 129 to 158 204.17.5.160 255.255.255.224 host address range 161 to 190 204.17.5.192 255.255.255.224 host address range 193 to 222 204.17.5.224 255.255.255.224 host address range 225 to 254
The network subnetting scheme in this section allows for eight subnets, and the network might appear as:
Figure 2
Notice that each of the routers in Figure 2 is attached to four subnetworks, one subnetwork is common to both routers. Also, each router has an IP address for each subnetwork to which it is attached. Each subnetwork could potentially support up to 30 host addresses.
This brings up an interesting point. The more host bits you use for a subnet mask, the more subnets you have available. However, the more subnets available, the less host addresses available per subnet. For example, a Class C network of 204.17.5.0 and a mask of 255.255.255.224 (/27) allows you to have eight subnets, each with 32 host addresses (30 of which could be assigned to devices). Si utiliza una máscara de 255.255.255.240 (/28), el desglose es:
204.17.5.0 - 11001100.00010001.00000101.00000000
255.255.255.240 - 11111111.11111111.11111111.11110000
--------------------------|sub |---
Since you now have four bits to make subnets with, you only have four bits left for host addresses. So in this case you can have up to 16 subnets, each of which can have up to 16 host addresses (14 of which can be assigned to devices).
Take a look at how a Class B network might be subnetted. Si tiene la red 172.16.0.0, entonces sabe que su máscara natural es 255.255.0.0 o 172.16.0.0/16. Extending the mask to anything beyond 255.255.0.0 means you are subnetting. You can quickly see that you have the ability to create a lot more subnets than with the Class C network. If you use a mask of 255.255.248.0 (/21), how many subnets and hosts per subnet does this allow for?
172.16.0.0 - 10101100.00010000.00000000.00000000
255.255.248.0 - 11111111.11111111.11111000.00000000
-----------------| sub |-----------
Usted utiliza cinco de los bits del host original para las subredes. Esto le permite tener 32 subredes (25). After using the five bits for subnetting, you are left with 11 bits for host addresses. Esto permite que cada subred tenga 2048 direcciones de host (211), de las cuales 2046 podrían asignarse a dispositivos.
Examples
Sample Exercise 1
Now that you have an understanding of subnetting, put this knowledge to use. In this example, you are given two address / mask combinations, written with the prefix/length notation, which have been assigned to two devices. Your task is to determine if these devices are on the same subnet or different subnets. Puede utilizar la dirección y la máscara de cada dispositivo para determinar a qué subred pertenece cada dirección.
DeviceA: 172.16.17.30/20 DeviceB: 172.16.28.15/20
Determine la subred del dispositivo A:
172.16.17.30 - 10101100.00010000.00010001.00011110
255.255.240.0 - 11111111.11111111.11110000.00000000
-----------------| sub|------------
subnet = 10101100.00010000.00010000.00000000 = 172.16.16.0
Looking at the address bits that have a corresponding mask bit set to one, and setting all the other address bits to zero (this is equivalent to performing a logical "AND" between the mask and address), shows you to which subnet this address belongs. In this case, DeviceA belongs to subnet 172.16.16.0.
Determine la subred del dispositivo B:
172.16.28.15 - 10101100.00010000.00011100.00001111
255.255.240.0 - 11111111.11111111.11110000.00000000
-----------------| sub|------------
subnet = 10101100.00010000.00010000.00000000 = 172.16.16.0
From these determinations, DeviceA and DeviceB have addresses that are part of the same subnet.
Sample Exercise 2
Given the Class C network of 204.15.5.0/24, subnet the network in order to create the network in Figure 3 with the host requirements shown.
Figure 3
Looking at the network shown in Figure 3, you can see that you are required to create five subnets. The largest subnet must support 28 host addresses. Is this possible with a Class C network? And if so, then how?
You can start by looking at the subnet requirement. In order to create the five needed subnets you would need to use three bits from the Class C host bits. Dos bits solo le permitirían cuatro subredes (22).
Since you need three subnet bits, that leaves you with five bits for the host portion of the address. How many hosts does this support? 25 = 32 (30 utilizables). This meets the requirement.
Therefore you have determined that it is possible to create this network with a Class C network. An example of how you might assign the subnetworks is:
netA: 204.15.5.0/27 host address range 1 to 30 netB: 204.15.5.32/27 host address range 33 to 62 netC: 204.15.5.64/27 host address range 65 to 94 netD: 204.15.5.96/27 host address range 97 to 126 netE: 204.15.5.128/27 host address range 129 to 158
VLSM Example
In all of the previous examples of subnetting, notice that the same subnet mask was applied for all the subnets. This means that each subnet has the same number of available host addresses. You can need this in some cases, but, in most cases, having the same subnet mask for all subnets ends up wasting address space. For example, in the Sample Exercise 2 section, a class C network was split into eight equal-size subnets; however, each subnet did not utilize all available host addresses, which results in wasted address space. Figure 4 illustrates this wasted address space.
Figure 4
Figure 4 illustrates that of the subnets that are being used, NetA, NetC, and NetD have a lot of unused host address space. Es posible que esto fuera una decisión de diseño pensando en el crecimiento futuro, pero en muchos casos se trata solo de espacio desperdiciado porque se emplea la misma máscara de subred para todas las subredes.
Variable Length Subnet Masks (VLSM) allows you to use different masks for each subnet, thereby using address space efficiently.
VLSM Example
Given the same network and requirements as in Sample Exercise 2 develop a subnetting scheme with the use of VLSM, given:
netA: must support 14 hosts netB: must support 28 hosts netC: must support 2 hosts netD: must support 7 hosts netE: must support 28 host
Determine what mask allows the required number of hosts.
netA: requires a /28 (255.255.255.240) mask to support 14 hosts netB: requires a /27 (255.255.255.224) mask to support 28 hosts netC: requires a /30 (255.255.255.252) mask to support 2 hosts netD*: requires a /28 (255.255.255.240) mask to support 7 hosts netE: requires a /27 (255.255.255.224) mask to support 28 hosts * a /29 (255.255.255.248) would only allow 6 usable host addresses therefore netD requires a /28 mask.
The easiest way to assign the subnets is to assign the largest first. For example, you can assign in this manner:
netB: 204.15.5.0/27 host address range 1 to 30 netE: 204.15.5.32/27 host address range 33 to 62 netA: 204.15.5.64/28 host address range 65 to 78 netD: 204.15.5.80/28 host address range 81 to 94 netC: 204.15.5.96/30 host address range 97 to 98
This can be graphically represented as shown in Figure 5:
Figure 5
Figure 5 illustrates how using VLSM helped save more than half of the address space.
CIDR
El enrutamiento entre dominios sin clases (CIDR) se introdujo para mejorar el aprovechamiento de espacio en las direcciones y la escalabilidad de enrutamiento en Internet. It was needed because of the rapid growth of the Internet and growth of the IP routing tables held in the Internet routers.
CIDR se aleja de las clases IP tradicionales (clase A, clase B, clase C, etc.). In CIDR , an IP network is represented by a prefix, which is an IP address and some indication of the length of the mask. Length means the number of left-most contiguous mask bits that are set to one. So network 172.16.0.0 255.255.0.0 can be represented as 172.16.0.0/16. CIDR also depicts a more hierarchical Internet architecture, where each domain takes its IP addresses from a higher level. This allows for the summarization of the domains to be done at the higher level. For example, if an ISP owns network 172.16.0.0/16, then the ISP can offer 172.16.1.0/24, 172.16.2.0/24, and so on to customers. Yet, when advertising to other providers, the ISP only needs to advertise 172.16.0.0/16.
Para más información sobre CIDR, vea RFC 1518 y RFC 1519.
Subredes especiales
Subredes de 31 bits
Una máscara de subred de 30 bits permite cuatro direcciones IPv4: dos direcciones host, una red de ceros y una dirección de broadcast de todo unos. Un link punto a punto sólo puede tener dos direcciones de host. No hay necesidad real de tener las direcciones broadcast y all-zeros con links punto a punto. Una máscara de subred de 31 bits permitirá exactamente dos direcciones de host, y elimina las direcciones de broadcast y de todo ceros, conservando así el uso de direcciones IP al mínimo para los links punto a punto.
Consulte RFC 3021: Uso de Prefijos de 31 Bits en Links Punto a Punto IPv4.
La máscara es 255.255.255.254 o /31.
La subred /31 se puede utilizar en links punto a punto verdaderos, como interfaces seriales o POS. Sin embargo, también se pueden utilizar en tipos de interfaz de broadcast como interfaces ethernet. Si ese es el caso, asegúrese de que sólo se necesitan dos direcciones IPv4 en ese segmento Ethernet.
Ejemplo:
192.168.1.0 y 192.168.1.1 están en la subred 192.168.1.0/31.
R1(config)#int gigabitEthernet 0/1
R1(config-if)#ip address 192.168.1.0 255.255.255.254
% Warning: use /31 mask on non point-to-point interface cautiously
La advertencia se imprime porque gigabitEthernet es un segmento de difusión.
Subredes de 32 bits
Una máscara de subred 255.255.255.255 (una subred /32) describe una subred con una sola dirección de host IPv4. Estas subredes no se pueden utilizar para asignar direcciones a los links de red, porque siempre necesitan más de una dirección por link. El uso de /32 está estrictamente reservado para el uso en links que pueden tener solamente una dirección. El ejemplo para los routers Cisco es la interfaz de loopback. Estas interfaces son interfaces internas y no se conectan a otros dispositivos. Como tal, pueden tener una subred /32.
Ejemplo:
interface Loopback0
ip address 192.168.2.1 255.255.255.255
Appendix
Configuración de muestra:
Routers A and B are connected via serial interface.
Router A
hostname routera ! ip routing ! int e 0 ip address 172.16.50.1 255.255.255.0 !(subnet 50) int e 1 ip address 172.16.55.1 255.255.255.0 !(subnet 55) int s 0 ip address 172.16.60.1 255.255.255.0 !(subnet 60) int s 0 ip address 172.16.65.1 255.255.255.0 (subnet 65) !S 0 connects to router B router rip network 172.16.0.0
Router B
hostname routerb ! ip routing ! int e 0 ip address 192.1.10.200 255.255.255.240 !(subnet 192) int e 1 ip address 192.1.10.66 255.255.255.240 !(subnet 64) int s 0 ip address 172.16.65.2 (same subnet as router A's s 0) !Int s 0 connects to router A router rip network 192.1.10.0 network 172.16.0.0
Host/Subnet Quantities Table
Class B Effective Effective # bits Mask Subnets Hosts ------- --------------- --------- --------- 1 255.255.128.0 2 32766 2 255.255.192.0 4 16382 3 255.255.224.0 8 8190 4 255.255.240.0 16 4094 5 255.255.248.0 32 2046 6 255.255.252.0 64 1022 7 255.255.254.0 128 510 8 255.255.255.0 256 254 9 255.255.255.128 512 126 10 255.255.255.192 1024 62 11 255.255.255.224 2048 30 12 255.255.255.240 4096 14 13 255.255.255.248 8192 6 14 255.255.255.252 16384 2 Class C Effective Effective # bits Mask Subnets Hosts ------- --------------- --------- --------- 1 255.255.255.128 2 126 2 255.255.255.192 4 62 3 255.255.255.224 8 30 4 255.255.255.240 16 14 5 255.255.255.248 32 6 6 255.255.255.252 64 2 *Subnet all zeroes and all ones included. These might not be supported on some legacy systems. *Host all zeroes and all ones excluded.
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