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work traffic.

There are different classes on networks. The largest is a class A, in which the entire network begins with the same number such as ’148.’, allowing up the last three numbers to be used for local network addresses. The next smaller network is class B, in which the first two numbers are designated, for example ’148.61.’. A class C network specifies the first three digits, such as ’148’61.104.’, and can have up to 256 addresses. The ’netmask’ indicates how many computers can be on a network. A common netmask is ’255.255.255.0’ which indicates the local network can have up to 256 computers. A netmask of ’255.255.255.254’ would indicate that there are only two computers on the network.

When a packet of information is sent it passes through many computers between the sender and receiver. Each of the computers is configured to know where the next computer is upstream and downstream. The ’gateway’ is the computer on a local area network that passes a packet out to the Internet. For example, if my computer address is 192.168.1.20, the gateway is probably 192.168.1.254. Any packets travelling to/from the Internet will travel through the gateway computer.

The current standard of four number network addresses is called IPV4. This addressing scheme is ultimately limited, and so the address space is being expanded from four numbers to six in the newer IPV6 standard. This standard also introduces some enhancements for security and other applications.

4.3.2 Computer Ports

On the network information is sent in packets. These are addressed to a computer using the IPV4 address, but they also include a port number between 0 and 65535. The port number indicates what service they are trying to access. In general the first 1000 are allocated to well known and agreed upon services, such as email and web serving. Other port numbers in the low thou-

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sands are for less common network services, and numbers in the high thousands are used by user

programs. An abbreviated list of common network ports is given below. On any computer the

may or may not be active.

23? 24? - telnet xxx - ftp

25 - smtp

80 - http

110 - pop3

118 - sqlserv

143 - imap

515 - printer

520 - router

1433 - mssql

26000 - quake

4.3.2.1 - Mail Transfer Protocols

Sending and receiving email involves different protocols. Mail is sent using a protocol called

Simple Mail Transfer Protocol (SMTP). Mail is retrieved with Post Office Protocol (POP) or

Internet Mail Access Protocol (IMAP). All of these protocols are handled with programs listening

on different ports on the server.

4.3.2.2 - FTP - File Transfer Protocol

File Transfer Protocol (FTP) is a very old and well supported method for transferring files

between computers. Advanced users will often use it with typed commands, but there are also

hundreds of graphical clients that hide the typed commands.

4.3.2.3 - HTTP - Hypertext Transfer Protocol

Hypertext Transfer Protocol (HTTP) is used for retrieving web pages from remote sites. It

uses simple commands to get text files from the remote computer.