Specifications

The current version of IP is version 4. Each node is assigned a 32-bit address, so the maximum population

of the Internet is 4 billion devices. This has been recognized as a serious limitation for some time and a new

version of IP version 6 expands the address space to 128 bits. This is a truly gigantic number. If IPv6

addresses were uniformly distributed over the Earth it would result in thousands of addresses per square

foot. Due to the scarcity of IPv4 addresses ISPs change extra for multiple addresses. Several techniques

have been developed to minimize the consumption of addresses.

IPv6 also promises to minimize the size of routing tables each router must maintain. While IPv6 holds

much promise it entails a complete overhaul of the Internet. Such change is always resisted until one has no

choice to go through the pain of conversion.

7.4.1 Dotted-Decimal Notation

Internet addresses are expressed in dotted decimal notation, four decimal numbers separated by periods,

nnn.nnn.nnn.nnn. The 32-bit address is divided into four 8-bit fields called octets. Each field has a range of

0-255. The smallest address is 0.0.0.0 and the largest 255.255.255.255.

7.4.2 Subnet

IP addresses consist of three components, the Network-Prefix, Subnet-Number and the Host Number. The

purpose of Subnetting is to allow IP addresses to be assigned efficiently and simplify routing.

For our purposes all the computers on a simple network must be on the same subnet. For example our

network allows up to 254 hosts (computers) the subnet mask is 255.255.255.0, also called a /24 subnet

because the first 24 bits are fixed.

7.4.3 Port Number

A single computer may connect to multiple hosts. How does the computer know how to interpret each

packet? For example, while writing this paper my mail program is checking e-mail, and I’m listening to a

Real Audio radio program. Each TCP or UDP packet includes a port number. Port numbers are 16 bit

values that range from 0-65,535. For example when you enter a URL into you web browser to access a

World Wide Web site the browser automatically uses port 80. The low port numbers 0-1023 are called the

well-known ports; they are assigned by IANA the Internet Assigned Number Authority when a particular

service is defined. Software uses that port to make initial contact. After the connection is established the

high numbered ports are used.

7.4.4 Private Addresses

The Internet Assigned Number Authority allocates Internet addresses. This is the entity that assigned the

addresses used by your ISP. RFC 1918 allocated three blocks of private addresses that are not used on the

Internet. The private addresses are ideal for use on a LAN. Devices on the LAN are assigned an address

from the pool of private addresses. This eliminates the need for coordination and the expense of obtaining

addresses from the ISP. When a computer on the LAN needs to access the Internet the gateway router uses

a technique called Network Address Translation (NAT) to convert the private IP addresses to the public

address assigned by the ISP.

Excerpt from IETF RFC 1918 Address Allocation for Private Internets:

The Internet Assigned Numbers Authority (IANA) has reserved the

following three blocks of the IP address space for private

Internets:

10.0.0.0 - 10.255.255.255 (10/8 prefix)

172.16.0.0 - 172.31.255.255 (172.16/12 prefix)

192.168.0.0 - 192.168.255.255 (192.168/16 prefix)