DHCP also helps in the assigning of IP addresses in large complex networks, where manual assigning is not an easy task. Introduction & History Dynamic Host Configuration Protocol, DHCP, is a standard protocol which is defined by RFC 1541. This allows a server on a local network to dynamically distribute IP addresses to its clients along with the configuration information (KB169289 1). The main purpose of the protocol is to reduce the work of the systems administrator of a large IP network.
Usually the DHCP server provides its client with information, which in the most basic form includes at least the IP address, Subnet Mask, and Default Gateway (KB169289 2). The most signification information of these is the IP address. DHCP, like BOOTP, also runs over UDP, utilizing ports 67 and 68 (Wobus 3). History DHCP is an extension of an earlier protocol called Bootstrap Protocol BOOTP. As the name suggests this protocol was used to pass information during initial booting to client systems in a network, and was designed to store and update static information including IP addresses.
BOOTTP was extended to DHCP, by the IETF i. e. the Internet Engineering Task Force, to manage the general dynamic configuration information. For standardizing the DHCP, IETF issued a series of RFCs focused on DHCP extensions the most recent of which is RFC 2131 issued in March 1997 (DHCP 5). Working DHCP allocates IP addresses, tracks the usage of these addresses, and reclaims a preset list of IP addresses and configuration information which is shared in a network of systems. When a client system is booted up, this system broadcasts a request to a DHCP server to issue it an IP address.
Usually the DHCP server responds with an IP address and for a specified leased time, for which the client may use the given address (DHCP 4). A typical server allows its administrator to set the lease time. Figure below gives a common topology of DHCP client-server environment (http://www. sun. com/software/whitepapers/solaris9/dhcp. pdf) In the above figure: a DHCP client is am IP host obtaining configuration via DHCP, a DHCP server is an IP host returning configuration to DHCP clients, a BOOTP Relay Agent is a router passing DHCP messages across networks.
Methods There are three methods by which information could be allocated: Manual, where the addresses are allocated manually by the systems administrator; Automatic, where the addresses are allocated from a pool and associated with a PC address unless there is a manual intervention; Dynamic, where the addresses allocated from a pool for a specified length of lease time (Knapp & Hadley 8) Uses -As mentioned above, DHCP is most commonly used to move the management of IP addresses away from distributed client systems and onto one or more centrally managed servers.
This mean that there is no need to configure TCP/IP parameters into client machines, and hence saves time required for configuring or debugging the network environment and in the process reduces the cost of ownership for client systems. DHCP is particularly useful in case of Networks with many more TCP/IP clients than network administrators, Networks where laptops commonly move among networks within the site, Networks which have fewer available TCP/IP addresses than the clients that need them e. g. in dial-up situation for ISP environments, Networks where the location of services is frequently moved from host to host, Networks supporting diskless clients. Etc. (DHCP 5) Main concerns with using DHCP There are some concerns when DHCP is used. The major issue is that the Current DHCP has no server-server protocol. IP address is almost always managed by a single server. This means multiple DHCP servers on a same subnet work fine, if their entire database is independent to each other.
There is also a scalability problem, meaning if many clients begin DHCP configuration at once, say after a power failure, there may be too many DHCP requests in a short time period. Current DHCP has little consideration about such a situation. Also there is a problem of linking DHCP and LDAP (Lightweight Directory Access Protocol) which has problems with Replication, Data sharing and Linkage to security server (Knapp & Hadley 20)
References Books Miller P,LAN technologies Explained, 2000, Digital Press Websites DHCP, Dynamic Host Configuration Protocol, http://www. sun. com/software/whitepapers/solaris9/dhcp. pdf KB169289, DHCP (Dynamic Host Communication Protocol Basics, Rev 3. 2, 27th Feb 2007, http://support. microsoft. com/kb/169289 Knapp L. J, Hadley T. M, Dynamic Host Configuration Protocol, http://www. lauraknapp. com/images/dhcp. pdf