Reference Manual for the ProSafe VPN Firewall FVS318v3
B-4 Network, Routing, and Firewall Basics
January 2005
• Class C
Class C addresses can have 254 hosts on a network. Class C addresses use 24 bits for the
network address and eight bits for the node. They are in this range:
192.0.1.x to 223.255.254.x.
• Class D
Class D addresses are used for multicasts (messages sent to many hosts). Class D addresses are
in this range:
224.0.0.0 to 239.255.255.255.
• Class E
Class E addresses are for experimental use.
This addressing structure allows IP addresses to uniquely identify each physical network and each
node on each physical network.
For each unique value of the network portion of the address, the base address of the range (host
address of all zeros) is known as the network address and is not usually assigned to a host. Also,
the top address of the range (host address of all ones) is not assigned, but is used as the broadcast
address for simultaneously sending a packet to all hosts with the same network address.
Netmask
In each of the address classes previously described, the size of the two parts (network address and
host address) is implied by the class. This partitioning scheme can also be expressed by a netmask
associated with the IP address. A netmask is a 32-bit quantity that, when logically combined (using
an AND operator) with an IP address, yields the network address. For instance, the netmasks for
Class A, B, and C addresses are 255.0.0.0, 255.255.0.0, and 255.255.255.0, respectively.
For example, the address 192.168.170.237 is a Class C IP address whose network portion is the
upper 24 bits. When combined (using an AND operator) with the Class C netmask, as shown here,
only the network portion of the address remains:
11000000 10101000 10101010 11101101 (192.168.170.237)
combined with:
11111111 11111111 11111111 00000000 (255.255.255.0)
Equals:
11000000 10101000 10101010 00000000 (192.168.170.0)
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