IP Addressing

Internet Protocol (IP) addresses are unique numeric identifiers that are needed by every device that connects to the Internet. The are a shared common resource that must be managed carefully to ensure the continued growth and stability of the Internet. Most of the Internet is currently addressed with IP version 4 (IPv4) addresses.

The Internet protocols consist of a suite of

communication protocols, of which the two best known are the Transmission Control Protocol

(TCP) and the Internet Protocol (IP). The Internet protocol suite not only includes lower-layer

protocols (such as TCP and IP), but it also specifies common applications such as electronic mail,

terminal emulation, and file transfer.

TCP/IP is based on a four-layer reference model. All protocols that belong to the TCP/IP protocol suite are located in the top three layers of this model.   Each layer is in charge of a set of network functions designed to move data across a network.

Layer	Description	Protocols
Network Interface	This layer specifies how data is physically transmitted across the network and includes specifications for the hardware that connects the device to the cable, the cable media itself and all the connecting hardware.	Ethernet, Serial, Frame Relay, ATM, etc.
Internet Layer	Sometimes called the network layer as the protocols in this layer are responsible for the sending and receiving the data on the network and for deciding how packets of data get from one network to another (called routing).	IP, ICMP, ARP
Transport Layer	This layer is responsible for ensuring reliable delivery of data from end to end and for other connection management activities.	TCP, UDP
Application Layer	This layer defines how TCP/IP applications talk to or use the transport layer in order to effectively communicate across the network.	HTTP, Telnet, FTP,  SNMP, DNS, SMTP, HTTP and many others

Any device or computer on a network may want to talk to any other device on that or any other connected network.  To achieve this every device connected to the network must have a unique address, just like using a postal service.  No two computers may have the same address as the device sending the data will not know which device to send it to. 

The TCP/IP suite of protocols uses the IP protocol to handle all addressing and routing issues.  An IP address is a unique number that identifies each host (computer or TCP/IP device) to the network.  The IP address is made up of 2 parts called the network ID and the host ID.  The network ID identifies the network on which the host is located.  The host ID is the identifier for the host on that network. 

The IP address is a 32 bit number.  An example of an IP address in binary would be.

IP Address Classes

The Internet community has defined five address classes. Class A, B, and C addresses are used for assignment to TCP/IP nodes.  Classes D and E are not typically used for nodes on the network.  The class of address defines which bits are used for the network and host ID parts of each address.  The address class also defines how many networks and hosts per network can be supported.  If we define a.b.c.d as an IP address then the table below shows the range of addresses, network and host portions of the IP address, number of networks and number of hosts per network for each class of IP address.

Class	Address Range of first octet	Network ID	Host ID	Number of Networks	Number of hosts per network
A	1 to 126	a	b.c.d	126	16777214
B	128 to 191	a.b	c.d	16384	65534
C	192 to 223	a.b.c	d	2097152	254
D	224 to 239	n/a	n/a	n/a	n/a
E	240 to 254	n/a	n/a	n/a	n/a

Thus IP address 192.168.1.3 is a class C address, its network ID is 192.168.1 and its host ID is 3.

The network and broadcast addresses for each network class are listed in the table below:

Class	Network Address	Broadcast Address
A	X.0.0.0	X.255.255.255
B	X.X.0.0	X.X.255.255
C	X.X.X.0	X.X.X.255

where X is a valid number for that address class.

Selecting a Network Address

When creating a new network you have several choices to make before assigning addresses to your computers and other IP nodes.  Firstly you should decide whether you need to use private IP addresses or real or Internet Assigned IP addresses.

Particular addresses in each address class have been reserved for private IP networks.  These address ranges are listed in the table below:

Class	Reserved Address	Addresses in this range
A	10.X.X.X	10.0.0.1 to 10.255.255.255
B	172.16.X.X to 172.31.X.X	172.16.0.0 to 172.31.255.255
C	192.168.X.X	192.168.0.0 to 192.168.255.255

These address ranges are designed for internal use and will not be routed through the Internet.  When designing a small to medium size network it is common to use the 192.168 series of addresses.

If you need routable or real Internet assigned IP addresses you should contact your ISP for a series of addresses.  When connecting your network to the Internet, typically you will only need a few real addresses for the devices that actually connect your network to the Internet and for web and email servers or firewalls.