DES-3326S Layer 3 Fast Ethernet Switch User’s Guide

This example uses an 11-bit subnet mask. (There are 3 additional bits added to the default Class A subnet mask). So the number of subnets is:

23 – 2 = 8 – 2 = 6

Subnets of all “0”s and all “1”s are not allowed, so 2 subnets are subtracted from the total.

The number of bits used in the node part of the address is 24 –

3 = 21 bits, so the total number of nodes is:

221 – 2 = 2,097,152 – 2 = 2,097,150

Multiplying the number of subnets times the number of nodes gives 12,582,900 possible nodes.

Note that this is less than the 16,777,214 possible nodes that an unsubnetted class A network would have.

Subnetting reduces the number of possible nodes for a given network, but increases the segmentation of the network.

Classless InterDomain Routing – CIDR

Under CIDR, the subnet mask notation is reduced to a simplified shorthand. Instead of specifying all of the bits of the subnet mask, it is simply listed as the number of contiguous “1”s (bits) in the network portion of the address. Look at the subnet mask of the above example in binary - 11111111.11100000.00000000.00000000 – and you can see that there are 11 “1”s or 11 bits used to mask the network address from the node address. Written in CIDR notation this becomes:

10.32.0.0/11

86

Switch Management and Operating Concepts

Page 86
Image 86
D-Link DES-3326S manual Classless InterDomain Routing Cidr