Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 274

Mask Nomenclature
IP subnet masks can be represented in several ways. The masks can be represented in binary,
hexadecimal, dotted-decimal, or bit mask. The most common way is the dotted decimal format
(255.255.255.0). Recently, the number of bits format has become popular. This format represents
the mask by using a slash with the number of bits with 1s in the mask. For example, 255.255.0.0
is represented as /16. Table 6-7 shows some mask representations.

Subnet Masks

Table 6-7

Dotted Decimal Bit Mask Hexadecimal

255.0.0.0 /8 FF-00-00-00

255.192.0.0 /10 FF-C0-00-00

255.255.0.0 /16 FF-FF-00-00

255.255.224.0 /19 FF-FF-E0-00

255.255.240.0 /20 FF-FF-F0-00

255.255.255.0 /24 FF-FF-FF-00

255.255.255.224 /27 FF-FF-FF-E0

255.255.255.240 /28 FF-FF-FF-F0

255.255.255.248 /29 FF-FF-FF-F8

255.255.255.252 /30 FF-FF-FF-FC

Determining the Network Portion of an IP Address
If you have an IP address and subnet mask, you can determine the full classful network,
subnetwork, and broadcast number of the subnetwork. This is accomplished by a logical
AND operation between the IP address and subnet mask. Table 6-8 shows the AND operation.
The AND operation is similar to the multiplication of Bit 1 and Bit 2; if any 0 is present, the
result is 0.

The AND Logical Operation Table 6-8

Bit 1 Bit 2 AND

0 0 0

0 1 0

1 0 0

1 1 1