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Network Considerations
Network Primer
I
I-7
The IP address provides two pieces of information: the network ID and the host
ID. The network ID must be unique among all network subnets within a connected
Internet. It identifies the network subnet to which a host is connected. The host ID
must be unique among hosts within a network subnet and identifies the host within
the subnet.
Network subnets connected to the public Internet must obtain an official network
ID from the Network Information Center (800-444-4345 or nic@nic.ddn.mil) to
preserve the unique network ID’s within the Internet. However, if you never intend
to connect to the Internet, you’re free to select your own network IDs.
The IP address consists of four numbers. Each of these four numbers can have a
value in the range 1 to 254. An IP address is written in the form w.x.y.z. For
example, "198.178.246.10" (or "C6.B2.F6.0A" in hexadecimal) is a valid IP
address.
There are three commonly used classes of IP address assignment as follows:
Class w Value Net ID Host ID
Available
Subnets
Available
Hosts
A 1 to 126 w x, y, z 126 16777214
B 128 to 191 w, x y, z 16384 65534
C 192 to 223 w, x, y z 2097151 254
The following are reserved addresses and may not be assigned to hosts:
127.0.0.0
128.0.0.0
191.255.0.0
192.0.0.0
223.255.255.0
w = 224 through 255 with x, y, and z equal to any value
Most IP subnets have Class B or C Net IDs because there are quite a few of these
available. You may be able to get a Class A Net ID, but there are so few of these
available (only 126 world-wide) that they are rarely issued and most are already
assigned.
Most companies get one or more Class C assignments, which means that there is a
limit of 254 hosts per network subnet.
The TCP/IP software separates the Net ID portion of the IP address from the Host
ID portion using the user-assigned Subnet Mask. This mask looks like an IP
address, but has a bit set in each bit position of the portion of the IP address that is
in the Net ID portion. For example, the Subnet Mask 255.255.255.0 sets the Net