Subnet Masks - Cisco Catalyst 6500 Series Configuration Manual

IP Addresses and Subnet Masks
Private Networks
If you need large numbers of addresses on your network, and they do not need to be routed on the
Internet, you can use private IP addresses that the Internet Assigned Numbers Authority (IANA)
recommends (see RFC 1918). The following address ranges are designated as private networks that
should not be advertised:
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Subnet Masks

A subnet mask lets you convert a single Class A, B, or C network into multiple networks. With a subnet
mask, you can create an extended network prefix that adds bits from the host number to the network
prefix. For example, a Class C network prefix always consists of the first three octets of the IP address.
But a Class C extended network prefix uses part of the fourth octet as well.
Subnet masking is easy to understand if you use binary notation instead of dotted decimal. The bits in
the subnet mask have a one-to-one correspondence with the Internet address:
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Example 1: If you have the Class B address 129.10.0.0 and you want to use the entire third octet as part
of the extended network prefix instead of the host number, you must specify a subnet mask of
11111111.11111111.11111111.00000000. This subnet mask converts the Class B address into the
equivalent of a Class C address, where the host number consists of the last octet only.
Example 2: If you want to use only part of the third octet for the extended network prefix, then you must
specify a subnet mask like 11111111.11111111.11111000.00000000, which uses only 5 bits of the third
octet for the extended network prefix.
You can write a subnet mask as a dotted decimal mask or as a /bits ("slash bits") mask. In Example 1,
for a dotted decimal mask, you convert each binary octet into a decimal number: 255.255.255.0. For a
/bits mask, you add the number of 1s: /24. In Example 2, the decimal number is 255.255.248.0 and the
/bits is /21.
You can also supernet multiple Class C networks into a larger network by using part of the third octet
for the extended network prefix. For example, 192.168.0.0/20.
Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Configuration Guide
D-2
10.0.0.0 through 10.255.255.255
172.16.0.0 through 172.31.255.255
192.168.0.0 through 192.168.255.255
The bits are set to 1 if the corresponding bit in the IP address is part of the extended network prefix.
The bits are set to 0 if the bit is part of the host number.
Appendix D Addresses, Protocols, and Ports Reference
OL-6392-01