Understanding the Dynamic Host Configuration Protocol (DHCP)

Understanding the Dynamic Host Configuration Protocol (DHCP)
	Chapter 5. Basic Network Configuration

In the early days of networks, IP addresses were manually assigned to individual hosts. This meant the network administrator had to visit individual hosts, assigning them unique IP addresses. This is fine if you are working in a small contained environment. However, as networks grew, controlling IP addresses became more and more unmanageable. Enter DHCP. Prior to DHCP, UNIX administrators used a protocol called the boot protocol (or bootp for short). This did not solve the problem of dynamically assigning IP address, but it was the forerunner to DHCP and is inter-operable with DHCP in environments which still require bootp.

DHCP offers the following benefits:

Host IP addresses can be assigned from a central place, lowering support costs when a new machine is installed or replaced.
It solves the problem of duplicate IP addresses. IP addresses on a network MUST ALWAYS be unique.
It solves major problems in the event of IP reshuffles.

	Note
	In the early days of the Internet, people applied to the InterNIC for a legal IP address class. Often they were granted it, and promptly used it around their organizations. Later, when InterNIC realized there was a shortage of IP addresses due to the exponential growth of the Internet, they had to put some mechanisms in place to stem the tide of the use of class A, B and C addresses. One of the ways to do this was to ask organizations to move to "illegal" addresses within their organizations and return blocks of legal IP addresses back to InterNIC. This obviously required many organizations to go through an IP address reshuffle - a time consuming and costly process. DHCP helped to solve some of these problems.

In this part of the course, we merely configure the client, however, in the Advanced Networking Course, there is a more extensive coverage of configuring the DHCP server.

DHCP is a broadcast service. By this I mean that the client will broadcast a DHCP request and wait for an answer. Assuming there is a server that is listening for DHCP requests, it will reply. In general, if there is more than one DHCP server, the first to answer the DHCP request will supply the IP address to the client - which of course might not be quite what you want. DHCP can also supply other parameters to the client including the default gateway, the DNS server, the length of time the client is allowed to retain this address, and, if required, a boot image for the client.

In conclusion, configuration of the client is as simple as we've seen earlier in the notes. Simply supplying the relevant parameter to the /etc/network/interfaces will dynamically obtain the IP address. Client configuration occurs using one of three pieces of software, namely, pump, dhcp-client (of dhclient), or dhcpcd. In it 's default form, the client works properly in it 's unconfigured state, however these programs have a configuration file that allows tweaking of the system.


Chapter 5. Basic Network Configuration		Chapter 6. Electronic Mail