Installation guide
Network Infrastructure for EtherNet/IP™
Understanding the Basics of Network Protocols
2-8
IP addressing supports three different network classes. Class A networks are intended mainly for use
with a few very large networks because they provide only 8 bits for the network address field. Class B
networks allocate 16 bits, and Class C networks allocate 24 bits for the network address field. Class C
networks provide only 8 bits for the host field, so the number of hosts per network may be a limiting
factor. In all three cases, the leftmost bit(s) indicate the network class. IP addresses are written in dotted
decimal format (e.g., 34.0.0.1.). Table 2-1 shows the address formats for Class A, B, and C IP networks.
Table 2-1 IP Address Formats for Class A, B, and C Networks
Class Leading Bits Start End
Class A 0 0.0.0.0 127.255.255.255
Class B 10 128.0.0.0 191.255.255.255
Class C 110 192.0.0.0 223.255.255.255
Class D (multicast
2
) 1110 224.0.0.0 239.255.255.255
Class E (reserved) 1111 240.0.0.0 255.255.255.255
IP networks can be divided into smaller units called
subnetworks or subnets. Subnets provide extra
flexibility for the network administrator. For example, assume that a network has been assigned a Class A
address, and all the nodes on the network use a Class A address. Further assume that the dotted decimal
representation of this network’s address is 34.0.0.0. (All zeros in the host field of an address specify the
entire network.) The administrator can subdivide the network using subnetting. This is done by
“borrowing” bits from the host portion of the address and using them as a subnet field.
If the network administrator has chosen to use 8 bits of subnetting, the second
octet of a Class A IP
address provides the subnet number. In our example, address 34.1.0.0 refers to network 34, subnet 1;
address 34.2.0.0 refers to network 34, subnet 2, and so on as shown in Figure 2-3.
Figure 2-3 Subdividing a Network.
The number of bits that can be borrowed for the subnet address varies. To specify how many bits are used
and where they are located in the host field, IP provides
subnet masks. Subnet masks use the same
format and representation technique as IP addresses as shown in Table 2-2.
2
A packet with a special destination address, which multiple nodes on the network may be willing to receive. Source: RFC 1983.
Internet Users’ Glossary. G. Malkin. August 1996.
http://rfc.net/rfc1983.html