Avaya Configuring IP Services Manual de usuario descargar pdf (Pagina 17)

IP Concepts, Terminology, and Features

1-5

With 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 inside 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:

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

• Allocates the 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.

You construct a subnet mask as follows:

• Assign a value of 1 to each of the 8, 16, or 24 bits in the network ﬁeld.

• Assign a value of 1 to each bit in the subnet ﬁeld.

• Assign a value of 0 to each bit in the host ﬁeld.

• Convert the resulting 32-bit string to dotted decimal notation.

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Avaya Configuring IP Services Manual de usuario Pagina 17