CTI 2572 Technical Overview Manual
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A typical IP address consists of a network ID portion and a host ID portion. The distribution of
the address bytes between network ID and host ID is defined by the address class. Class A uses
the first byte for the network ID and the remaining three bytes for the host ID. Class B uses two
bytes for the network ID and two bytes for the host ID. Class C uses the first three bytes for the
network ID and the last byte for the host ID. The class type is determined by the first few bits of
the address. Address classes provide the flexibility to handle both large and small networks
while conserving IP addresses. There will be a few networks with a large number of hosts (Class
A) and a large number of networks with a small number of hosts (Class C). Some addresses are
reserved for special functions, such as broadcast messages.
Address Class Byte 1 Byte 2 Byte 3 Byte 4
A Network ID Host ID Host ID Host ID
B Network ID Network ID Host ID Host ID
C Network ID Network ID Network ID Host ID
For a private network using internet protocol, assignment of network/node addresses is up to the
individual organization. Organizations with networks connected to the Internet (with a capital I)
must obtain the network portion of the IP addresses from a central authority - the Network
Information Center. The Host ID portion of the IP address is assigned by the local organization.
Thus the Host ID is often called the local portion of the IP address.
Subnet Address Mask
Larger organizations with many internal networks (subnets) may choose to subdivide the local
portion of the IP address into a subnet identification and a host ID. For example, an organization
with a class B address may choose to use byte 3 as the subnetwork ID and byte 4 as the host ID.
The subnetting scheme is defined by a 32 bit Subnet Mask. The mask is set to zeroes in that
portion of the address that represents the host ID and is set to ones everywhere else (network and
subnetwork portion). A subnet mask for the above example would appear as:
Byte 1 Byte 2 Byte 3 Byte 4
11111111 11111111 11111111 00000000
In dotted decimal notation this example would be expressed as 255.255.255.0
Gateways use the IP address to route IP datagrams from the sender host to the destination host.
If the hosts are on different networks, the gateway reads the network ID portion of the address
and determines how to forward the datagram so that it reaches its destination. In a large internet,
