ATL Telecom AM30 - Manual (Page 158)

158 ATL Telecom User Guide
AM30
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 different 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 government agency.
Class C networks are the smallest, only able to hold 254 hosts at most, but the total
possible number of class C networks 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:
fThe class can be determined easily from field1:
field1 = 1-126: Class A
field1 = 128-191: Class B
field1 = 192-223: Class C
(field1 values not shown are reserved for special uses)
fA host ID can have any value except all fields set to 0
or all fields set to 255, as those values are reserved
for special uses.
Subnet masks
Definition
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 define 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 identifies 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 field1 through field 3 are part of the network
ID, but note how the mask specifies that the first bit in field 4 is also included. Since this
extra bit has only two values (0 and 1), this means there are two subnets. Each subnet uses
the remaining 7 bits in field4 for its host IDs, which range from 0 to 127 (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 field4 can have four values (00, 01, 10, 11), so there are four subnets.
Each subnet uses the remaining six bits in field4 for its host IDs, ranging from 0 to 63.