User Guide

Page 180 SonicWALL Internet Security Appliance User’s Guide
• POP3 - Post Office Protocol 3 (POP3) is used to receive e-mail messages and storing
messages on a server, referred to as a POP server.
• ICMP - Internet Control Messages Protocol (ICMP) reports errors and controls
messages on a TCP/IP network. PING uses ICMP protocol to test if a network device is
available.
IP Addressing
To become part of an IP network, a network device must have an IP address. An IP address
is a unique number that differentiates one device from another on the network to avoid
confusion during communication. To help illustrate IP addresses, the following sections
compare an IP address to the telephone numbering system, a system that is used every
day.
Like a phone number with its long distance “1” and area code, an IP address contains a set
of four numbers. While we separate phone number components with dashes, for example
1-408-555-1212, IP address number components are separated by decimal points or dots
(called dotted decimal notation), for example 123.45.67.89. Because computers use a
binary number system, each number in the set must be less than 255.
There are three components of IP addressing:
• IP address
• Subnet mask
• Default gateway
IP Address
Just as each household or business requires a unique phone number, a networked device
(such as a computer, printer, file server, or router) must have a unique IP address. Unlike
phone numbers, an IP address requires the entire number when communicating with other
devices.
There are three classes of IP addresses: A, B, and C. Like a main business phone number
that one can call, and then be transferred through interchange numbers to an individual’s
extension number, the different classes of IP addresses provide for varying levels of
“interchanges” or subnetworks, and “extensions” or device numbers. The classes are based
on estimated network size:
• Class A — used for very large networks with hundreds of subnetworks and thou-
sands of devices. Class A networks use IP addresses between 0.0.0.0 and
127.0.0.0.
• Class B — used for medium to large networks with 10–100 subnetworks
and hundreds of devices. Class B networks use IP addresses between 128.0.0.0
and 191.0.0.0.
• Class C — used for small to medium networks, usually with only a few subnetworks
and less than 250 devices. Class C networks use IP addresses between 192.0.0.0
and 223.0.0.0.
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