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Dynamic Host Configuration Protocol (DHCP) is an English term indicating a Protocole network whose role is to ensure the automatic configuration of parameters IP of a Station, in particular by automatically assigning a Adresse IP and a to him Masque of sub-network. DHCP can also configure the address of the footbridge by defect, the waiters of names DNS and of the waiters of names NBNS (known under the names of waiters WINS on the networks of the Microsoft company).

The initial design of IP supposed the preconfiguration of each computer connected to the network with adequate parameters TCP/IP: it is static addressing . On large-sized or extended networks, where modifications often intervene, static addressing generates a heavy load of maintenance and risks of errors. Moreover the assigned addresses cannot be used even if the computer which holds it is not in service: case typical where this poses problem is that of suppliers of access to Internet (FAI or English ISP), who in general has more customers than IP addresses at their disposal, but all the customers are never connected at the same time.

DHCP brings a solution to these two disadvantages:

Seuls the computers in service use an address of the space of addressing;
Any modification of the parameters (addresses footbridge, waiters of names) is reflected on the stations during the restarting;
the modification of these parameters is centralized on waiters DHCP.

The protocol was presented for the first time in October 1993 and is defined by RFC1531, modified and supplemented by the RFC 1534, RFC 2131 and RFC 2132.

This protocol can function with IPv4; it also functions with IPv6 (DHCPv6), however in IPv6, the addresses can be autoconfigurées without DHCP.

Operation

the computer equipped with TCP/IP, but deprived of Address IP, sends by diffusion a Datagramme (DHCP DISCOVER) which is addressed to the port 67 of any waiter to listening on this port. This datagram comprises inter alia the addresses physical (MAC) of the customer.
Any waiter DHCP having received this datagram, if it is able to propose an address (DHCP OFFER) on the network to which the customer belongs, diffuses an offer DHCP for submission to the customer (on his port 68), identified by his physical address. This offer comprises address IP of the waiter, as well as address IP and the mask of sub-network which he proposes to the customer. It may be that several offers are addressed to the customer.
customer retains one of offers received (the first which reaches him), and diffuses on the network a datagram of request DHCP (DHCP REQUEST). This datagram comprises the address IP of the waiter and that which has just been proposed to the customer. It causes to ask the selected waiter the assignment of this address, the possible sending of the values of the parameters, and to inform the other waiters which made an offer that it was not retained.
selected waiter DHCP works out a datagram of acknowledgment of delivery ( DHCP ack ) which assigns with the customer address IP and its mask of sub-network, the duration of the lease of this address, two values T1 and T2 which determine the behavior of the customer at the end of the lease, and possibly of other parameters:

IP address of the footbridge by defect
IP addresses of waiters DNS
IP addresses of waiters NBNS (WINS)

The customer can also receive a type of node NETBIOS.

The list of the options which waiter DHCP can accept is consultable in the RFC 2132: Options DHCP and Extensions supplier BOOTP, Chapter RFC 1497: Extensions supplier.

Waiters DHCP, of course, must be equipped with a static address IP.

Compatibility

Many an OS can make turn of the customers dhcp v4. However Windows 2000 and Windows xp do not have dhcpv6. Dhcpv6 is available in Windows Vista. A waiter dhcpv6 is available in Windows 2008.

Renewal of the lease

Dynamic IP addresses are granted for one limited time, which is transmitted to the customer in the acknowledgment of delivery which encloses transaction DHCP.

The value T1 which accompanies it determines the duration after which the customer starts periodically to ask for the renewal of his lease near the waiter which granted its address to him (usually half of the duration of the lease). This time the transaction is carried out by traditional transmission IP, of address at address.

If when the time allowed by the second value, T2, is past, lease has not been able to be renewed (for example if waiter DHCP of origin is except service), the customer asks for a new allowance of address by diffusion.

So at the end of the lease the customer could neither obtain the renewal from it, nor to obtain a new allowance, the address is decontaminated and it loses faculty to use network TCP/IP in a normal way.

Customer and waiter on different segments

When the waiter DHCP and the customer are not reproduced on same the segment IP, the diffusions emitted by this last do not arrive at the waiter because the Routeur S, do not transmit the general diffusions (the RFC 1542 described the possibility for a router of letting pass diffusions DHCP). In this case one uses an agent of relay DHCP.

This particular host is configured with a static address IP, and knows the address of a waiter DHCP to which it transmits the requests DHCP which reach him on port 68 (listened by the program agent of relay). He diffuses on his segment (which is also that of the customer) the answers which he receives from waiter DHCP.

Configuration of waiter DHCP

So that a waiter DHCP can serve as IP addresses, it is necessary to give him a “tank” of addresses from which it will be able to draw: it is the beach of addresses (scope) . It is possible to define several beaches, disjoined or contiguous.

The addresses of the segment which do not appear in any beach availability of waiter DHCP will not be to in no case distributed, and can usually be the subject of static assignments (: for the waiters requiring an address IP fixes, the routers, the printers network…).

It is also possible to exclude for a use in static addressing for example, addresses or blocks of addresses included/understood in a beach.

Lastly, one can carry out reservations addresses by limiting the possibility of granting of this address to the customer having a given physical address. This can prove to be useful for machines whose address must remain fixed but which one wants to control in central manner and automatic other parameters IP.

During the use on the same segment of several waiters DHCP, the intersection of the beaches of addresses of the various waiters must be empty, under penalty of ambiguity in the assignments and the renewals. Indeed waiters DHCP do not exchange any relative information with the beams which they grant.

See too

Address IP
ARP/RARP
BOOTP

External bonds

Operation of protocol DHCP
the DHCP under Linux (customer and waiter)
RFC3736: Dynamic Host Configuration Protocol (DHCP) Service for IPv6
DHCP in detail with decomposition of the headings
protocol DHCP

Simple: Dynamic Host Configuration Protocol

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