3Com CoreBuilder 3500, Implementation Manual

Page: 267 / 592

Key Concepts 267
Figure 45
Extending the Network Prefix
Using the Class B IP address from our example (158.101.230.52), the
subnet mask is 255.255.255.240.
The number that includes both the Class B natural network mask
(255.255) and the subnet mask (255.240) is sometimes called the
extended network prefix .
Continuing with the previous example, the subnet part of the mask uses
12 bits, and the host part uses the remaining 4 bits. Because the octets
are actually binary numbers, the number of subnets that are possible with
this mask is 4,094 (2
12
), and the number of hosts that are possible in each
subnet is 16 (2 4 ).
Subnet Mask Numbering
There is an alternate method to represent the subnet mask numbers. This
method is based on the fact that the subnet mask numbers are based on
the number of bits that signify the network portion of the mask. Many
Internet Service Providers (ISP) providers now use this notation to denote
the subnet mask. See Table 39.
The subnet mask 255.255.255.255 is reserved as the default broadcast
address.
Table 39
Subnet Mask Notation
Standard Mask Notation Network Prefix Notation
100.100.100.100 (255.0.0.0) 100.100.100.100/8
100.100.100.100 (255.255.0.0) 100.100.100.100/16
100.100.100.100 (255.255.255.0) 100.100.100.100/24
IP address Network Subnet and Host
Subnet mask
networ Host subn
Subnet
Network
Apply the subnet mask
Result = subnet/host boundary
Take the IP address
1 0 1 1 1 1 1 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1