Avaya Configuring IP, ARP, RARP, RIP, and OSPF Services Instrukcja Użytkownika Pobierz pdf (Strona 26)

Configuring IP, ARP, RARP, RIP, and OSPF Services

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308627-14.20 Rev 00

Subnet Addressing

The concept of subnetworks (or subnets) extends the IP addressing scheme.

Subnets are two or more physical networks that share a common

network-identification field (the NIC-assigned network portion of the 32-bit IP

address). Subnets allow an IP router to hide the complexity of multiple LANs

from the rest of the internetwork.

Using subnets, you partition the host portion of an IP address into a subnet

number and a “real” host number on that subnet. The IP address is then defined by

network.subnet.host. Routers outside the network do not interpret the subnet and

host portions of the IP address separately.

Routers within a network containing subnets use a 32-bit subnet mask that

identifies the extension bits. In network.subnet.host, the subnet.host portion (or

the local portion) contains an arbitrary number of bits. The network administrator

allocates bits within the local portion to subnet and host, and then assigns values

to subnet and host.

For example, the following is the IP address of a network that contains

subnets: 10000000 00100000 00001010 10100111. You specify this address in

dotted-decimal notation as 128.32.10.167.

The second bit of the first octet is set to 0, indicating that the network is a Class B

network. Therefore, the NIC-assigned network portion contains 16 bits and the

locally assigned local portion contains 16 bits.

The network administrator allocates the 16 bits in the local portion field as

follows:

• Upper 8 bits (00001010) with a value of 10 to the subnet portion

• Lower 8 bits (10100111) with a value of 167 to the host portion

In other words, the 16-bit local portion field, together with the 16-bit network

field, specify host 167 on subnet 10 of network 128.32.

You now need a subnet mask to identify those bits in the 32-bit IP address that

specify the network field and those bits that specify the subnet field. Like the IP

address, you specify the subnet mask in dotted-decimal notation.

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Avaya Configuring IP, ARP, RARP, RIP, and OSPF Services Instrukcja Użytkownika Strona 26