DHCP

12/15/2001

DHCP: Dynamic Host Configuration Protocol. DHCP is a service that allocates IP addresses dynamically. It is widely used in networking in order to reduce the burden of allocating and maintaining unique IP addresses for every computer that might conceivably use the Internet. The 32 bit addressing scheme used by IP4 allows for -- at most -- ten thousand million unique addresses, but the methods used to allocate the addresses ensure that the address space can not be very densely populated. In practice, there aren't enough addresses to give every device a unique, permanent IP address. Neither is there an adequate mechanism to assign them except in rather large blocks. The answer to insufficient permanent addresses is to lease temporary addresses. DHCP is the method for doing that.

DHCP is widely used in corporate settings, by internet service providers and by proxy servers such as Microsoft's Internet Connection Service.

The major technical hurdle that DHCP must overcome is that lacking an IP address initially, a DHCP client can't use the TCP/IP packet to negotiate an address lease. Instead, it must send a special DHCP Service request that includes the client hardware address. DHCP servers respond to the request. The client then selects a DHCP server and requests an IP address which it uses. The DHCP server can provide additional information at the time it provides the IP addresses including addresses of Domain Name Service servers, WINS servers, etc. DHCP provided addresses are leased. The lease can be terminated by either the server or client and will expire after a time in any case.

Because DHCP provided addresses can and do change, it is difficult or impossible to use DHCP to provide IP addresses to servers that must be reached using the Internet. Conceptually at least, local DHCP assigned addresses can be found using a local Name Server. But potential external clients can't find those addresses using global Domain Name Servers. Lease periods are generally a few days. Renegotiation is undertaken early -- halfway through the lease period. The lease period is a compromise between a desire not to disrupt IP traffic by changing the address and the need to reuse IP addresses fairly promptly when a client shuts down or negotiates a new lease with a different server.

DHCP requests and responses can be propagated through routers and switches provided that the latter are configured to forward the closely related BOOTP protocol. BOOTP is a protocol used to bootstrap an operating system onto a (usually diskless) computer via a network.

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