Overview of Computer Networks

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Here we have three individual LANs, labeled A, B and C, tied together in an internet. (Note the lowercase

'i'.) The address names of the nodes here, such as saturn.xyz.com, use the IP format which will be

discussed later. This internet should not be confused with "the Internet," distinguished by the capital 'I'.

The Internet is a special internet connecting TCP/IP networks worldwide.2

Each node here is a computer, connected to one or more networks via one or more network interface cards

(NICs). Recall that in any computer, each I/O device's interface has a specific port number or address.

The keyboard interface, for instance, might be port 50. Whenever a program, usually the operating system

(OS), reads from the keyboard, the value 50 would go onto the computer's address bus. Similarly, a NIC has

an address too, say 60. Whenever the OS reads from or writes to the LAN, it does so via the NIC, using the

NIC's assigned address.

This can be confusing, because there will be several addresses at work here:

• The NIC address from the CPU's point of view is 60, in the sense that the NIC is connected to the

computer's address bus at that address.

• The NIC also has a LAN address, typically a 48-bit Ethernet ID number hardwired into the NIC at the

time of manufacture and unique among all NICs in the world of that LAN type.

• If the computer is part of the Internet, the NIC will be assigned an Internet (IP) address, a 32-bit

identifier unique among all Internet hosts in the world.3

• At the TCP layer of the network software, we also speak of ports which are purely software IDs that

have nothing to do with port numbers like 60 above. In essence, each TCP port is a different element in

a large array in the TCP software, each element corresponding to a different service such as sendmail

(a program that actually puts your outgoing e-mail onto the network, and receives incoming mail for

you, as opposed to the program you use to compose and read mail with), ftp, ssh and so on.

2Another similar term is intranet, which refers to a private collection of networks, say within a particular company, which we

do not wish to make accessible worldwide.

3Extended to 128 bits in IP version 6.

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Suppose I buy an Ethernet card and install it in saturn, but later swap it with the one in citroen. Then the

card's Ethernet ID will still be the same, but its IP address will change, and maybe its I/O port number

(previously 60) will change too.

A computer can be connected to more than one network, as is the case with mars.xyz.com here. For each

NIC a given computer has, the computer would answer to a different name. In addition, a computer can even

have more than one name associated with the same NIC. Here the machine has the names mars.xyz.com and

xyz.com associated with the interface to network A, and its names abc.com and jeep.abc.com are associated

with network B.