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IP Basic Address Structure and Main Components: Network ID and Host ID
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As I mentioned in the IP addressing
overview, one of the ways that IP addresses are used is to facilitate
the
routing of datagrams in an IP internet.
This is made possible because of the way that IP addresses are structured,
and how that structure is interpreted by network routers.
Internet IP Address Structure
As
we just saw, each version 4 IP address
is 32 bits long. When we refer to the IP address we use a dotted-decimal
notation, while the computer converts this into binary. However, even
though these sets of 32 bits are considered a single “entity”,
they have an internal structure containing two components:
- Network Identifier (Network ID): A certain
number of bits, starting from the left-most bit, is used to identify
the network where the host or other network interface is located. This
is also sometimes called the network prefix or even just the
prefix.
- Host Identifier (Host ID): The remainder
of the bits are used to identify the host on the network.
 Note: By convention, IP devices are often called hosts for simplicity, as I do throughout this Guide. Even though each host usually has a single IP address, remember that IP addresses are strictly associated with network-layer network interfaces, not physical devices, and a device may therefore have more than one IP address.
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As you can see in Figure 57,
this really is a fairly simple concept; it's the same idea as the structure
used for phone numbers in North America. The telephone number (401)
555-7777 is a ten-digit number usually referred to as a single “phone
number”. However, it has a structure. In particular, it has an
area code (“401”) and a local number (“555-7777”).
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