NetBSD rules - Go to the first, previous, next, last, above section, table of contents.

2.2 TCP/IP address format

TCP/IP uses 4-byte (32-bit) addresses in the current implementations (v4), also called IP-numbers (Internet-Protocol numbers).

Those IP-numbers are worldwide unique. To assure this, they are administrated by one central organisation, the InterNIC. They give certain ranges of addresses (network-addresses) directly to sites which want to participate in the internet or to internet-providers, which give the addresses to their customers.

If your university or company is connected to the Internet, it has (at least) one such network-address for it's own use.

If you just want to run your private network at home, see below on how to "build" your own IP-numbers. However, if you want to connect your machine to the (real :-) Internet, you should get an IP-number from your local network-administrator or -provider.

When writing down IP-numbers, this is done in "dotted quad"-notation most of the time, i. e. the four bytes are written down in decimal (MSB first), separated by dots. For example, would be a valid address. Another way to write down IP-addresses would be as one 32-bit hex-word, e.g. 0x84c70f63. This is not as convenient as the dotted-quad, but quite useful at times, too. (See below!)

Being assigned a network means nothing else but setting some of the above-mentioned 32 address-bits to certain values. These bits that are used for identifying the network are called network-bits. The remaining bits can be used to address hosts on that network, therefore they are called host-bits.

In the above example, the network-address is (host-bits are set to 0 in network-addresses), the host's address is 15.99 on that network.

How do you know that the host's address is 16 bit wide? Well, there are four classes of networks. Each one starts with a certain bit-pattern identifying it. Here are the four classes:

Please note that the bits which are used for identifying the network-class are part of the network-address.

When seperating host-addresses from network-addresses, the "netmask" comes in handy. In this mask, all the network-bits are set to "1", the host-bits are "0". Thus, putting together IP-address and netmask with a locical AND-function, the network-address remains.

To continue our example, is a possible netmask for When applying this mask, the network-address remains.

By default, every network-class has a fixed netmask assigned:

Class A:
default-netmask:, first byte of address: 1-127
Class B:
default-netmask:, first byte of address: 128-191
Class C:
default-netmask:, first byte of address: 192-223

Another thing that should be mentioned here is the "broadcast-address". When sending to this address, all hosts on the corresponding network will receive the message sent. The broadcast address is characterized by having all host-bits set to "1".

Taking with its netmask again, the broadcast-address would result in

You'll ask now: But what if I want a hosts address to be all bits "0" or "1"? Well, this doesn't work, as network- and broadcast-address must be present! Because of this, a class B network can contain at most 2^16-2 hosts, a class C network can hold no more than 2^8-2 = 254 hosts.

Besides all those categories of addresses, there's the special IP-address which always refers to the "local" host, i. e. if you talk to you'll talk to yourself without starting any network-activity. This is sometimes useful to use services installed on your own machine or to play around if you don't have other hosts to put on your network.

Let's put together the things we've introduced in this section:

NetBSD rules - Go to the first, previous, next, last, above section, table of contents.