TCP/IP Configuration and Management Manual

NonStop TCP/IP Processes and Protocols
TCP/IP Configuration and Management Manual—427132-004
B-6
The Problem of Resolving Addresses
To address subnets within your LAN, your LAN administrator can further divide the
local address part of your Internet addresses into a subnet number (identifying a
particular subnet) and a host number (uniquely identifying a host system within the
subnet).
To identify what part of the Internet address represents the subnet number, a 32-bit
subnet mask is used. All bits corresponding to the network address and its subnet
address are set to 1, and all bits corresponding to the remainder of the local address
are set to 0.
Figure B-2 shows an example of a Class B address in which the first five bits of the
local address are the subnet number, leaving the last eleven bits to identify the host.
As shown, the subnet mask consists of twenty-one 1's (corresponding to sixteen bits of
network address and five bits of subnet address) followed by eleven 0's.
Some of the remaining bits of the local address can define further subnets within each
first-level subnet, as shown in Figure B-3. In this example, the 11-bit host number from
Figure B-2 is subdivided into a 3-bit subnet number and an 8-bit host number. The
subnet mask at this level consists of twenty-four 1s and eight 0s.
Figure B-2. Subnet Mask
Figure B-3. Addressing of Nested Subnet
016VST .VSD
Network Address
Bit
Subnet
Mask
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0 0 0 0 0
01516202131
Subnet
Number
Host
Number
017VST .VSD
Network Address
Bit
Internet
Address
Subnet
Mask
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Host
Number
0 0 0 0 0 0 0 0
0151620213123 24
1 1 1
Level-1
Subnet
Number
Level-2
Subnet
Number