User`s manual

111
WLAN Router
Network classes
The three commonly used network classes are A, B, and C. (There is also a class D but it has a special use beyond the
scope of this discussion.) These classes have dierent uses and characteristics.
Class A networks are the Internet’s largest networks, each with room for over 16 million hosts. Up to 126 of these
huge networks can exist, for a total of over 2 billion hosts. Because of their huge size, these networks are used for
WANs and by organizations at the infrastructure level of the Internet, such as your ISP.
Class B networks are smaller but still quite large, each able to hold over 65,000 hosts. There can be up to 16,384 class
B networks in existence. A class B network might be appropriate for a large organization such as a business or go-
vernment agency.
Class C networks are the smallest, only able to hold 254 hosts at most, but the total possible number of class C ne-
tworks exceeds 2 million (2,097,152 to be exact). LANs connected to the Internet are usually class C networks.
Some important notes regarding IP addresses:
The class can be determined easily from eld1: •
eld1 = 1-126: Class A
eld1 = 128-191: Class B
eld1 = 192-223: Class C
(eld1 values not shown are reserved for special uses)
A host ID can have any value except all elds set to 0 or all elds set to 255, as those values are reserved for •
special uses.
Subnet masks
Denition
MASK
A mask looks like a regular IP address, but contains a pattern of bits that tells what parts of an IP address are
the network ID and what parts are the host ID: bits set to 1 mean “this bit is part of the network ID” and bits
set to 0 mean “this bit is part of the host ID.”
Subnet masks are used to dene subnets (what you get after dividing a network into smaller pieces). A subnet’s network ID
is created by “borrowing” one or more bits from the host ID portion of the address. The subnet mask identies these host ID
bits. For example, consider a class C network 192.168.1. To split this into two subnets, you would use the subnet mask:
255.255.255.128
It’s easier to see what’s happening if we write this in binary:
11111111. 11111111. 11111111.10000000
As with any class C address, all of the bits in eld1 through eld3 are part of the network ID, but note how the mask
species that the rst bit in eld4 is also included. Since this extra bit has only two values (0 and 1), this means the-
re are two subnets. Each subnet uses the remaining 7 bits in eld4 for its host IDs, which range from 1 to 126 hosts
(instead of the usual 0 to 255 for a class C address).
Similarly, to split a class C network into four subnets, the mask is:
255.255.255.192 or 11111111. 11111111. 11111111.11000000
The two extra bits in eld4 can have four values (00, 01, 10, 11), so there are four subnets. Each subnet uses the re-
maining six bits in eld4 for its host IDs, ranging from 1 to 62.
Note
Sometimes a subnet mask does not specify any additional network ID bits, and thus no subnets. Such a
mask is called a default subnet mask. These masks are:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
These are called default because they are used when a network is initially congured, at which time it has
no subnets.