13-3Lesson 1 Configuring and Troubleshooting TCP/IP■
The host ID is the portion of the IP address that identifies a particular host on a
network segment. The host ID for each host must be unique within the network
ID. Continuing the example of the IP address 192.168.1.102 (where 192.168.1.0 is
the network ID), the host ID is 102.
Two computers with different network IDs can have the same host ID. However, the
combination of the network ID and the host ID must be unique to all computers in
communication with each other.
Hosts depend on a second number called a subnet mask to help determine which
portion of an IP address is the network ID and which portion is the host ID. The subnet
mask defines where the network ID stops and the host ID starts. It is easier to see why
this works if you step away from the decimal representation for a moment and look at
the numbers in their binary format.
Figure 13-1 depicts a single IP address shown in both decimal and binary format. A
subnet mask is also shown in both formats. In binary format, a subnet mask always
represents a string of unbroken ones followed by a string of unbroken zeroes. The
position of the change from ones to zeroes indicates the division of network ID and
host ID in an IP address.
DecimalBinary00101010000011110110110110000111
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