Chapter30 More About....
IP Addresses and Subnet Masks
30-2
Cisco Router and Security Device Manager Version 2.2 User’s Guide
OL-4015-08
The subnet mask is used to specify how many of the 32 bits are used for the
network number and, if subnetting is used, the subnet number. It is a binary mask
with a 1 bit in every position used by the network and subnet numbers. Like the
IP address, it is a 32-bit value, expressed in decimal format. The following figure
shows a subnet mask entered in SDM. SDM shows the subnet mask and the
equivalent number of bits in the mask.
These values entered SDM represent the binary mask shown in the following
figure:
This subnet mask specifies that the first 24 bits of the IP address represent the
network number and subnet mask, and that the last 8 bits represe nt the host
number within that network and subnet. You can enter the mask in the dotted
decimal format shown in the Subnet Mask field, or you can select the n umber of
bits in the bits field. When you enter or select a value in one field, SDM
automatically adjusts the other.
SDM displays a warning window if you enter a decimal mask that results in binary
zeros (0s) in the network/subnet area of the mask. The following subnet mask
field contains a decimal value that would result in binary zeros in the
network/subnet number portion of the mask. Note that the bits field on the right
is empty, indicating that an invalid value has been entered in the Subnet Mask
field.