DHCP – Dynamic Host Control Protocol

DHCP refers to Dynamic Host Control Protocol. DHCP is responsible for dynamically assigning IP addresses to all the computers in a network immediately after the computer starts. Let us learn something more about DHCP, in this article.

What is DHCP?

As mentioned above, a DHCP (Server) is used to automatically allocate IP addresses for all the computers in a computer network. When a computer / network device (client) is switched on, it automatically sends a broadcast message to discover the DHCP Server. Once the DHCP Server gets the broadcast message, it sends an offer (IP address + configuration details) to the client. When the client accepts it, the client sends an accept message and the server responds back with an acknowledgement message containing the duration for which the IP address has been leased to the client.

So, a DHCP Server not only assigns the IP address to the client but it also sends some configuration parameters like subnet mask, default gateway, domain name, name servers, NTP (Network Time Protocol) information, etc. IP addresses are generally leased to the clients for a certain duration and this duration can be set by the administrator.

DHCP Server is included in the Network Server software packages like Windows 2008, Linux Network Server, etc. Many devices come with in-built DHCP servers, especially for assigning IP addresses for wireless clients. Example – Wireless Controllers, Access Points, Routers, etc.

Why is a DHCP Server required?

  • To avoid manual entry of IP address and configuration information for all the computers/ network devices individually.
  • To prevent two computers/ network devices from having the same IP address (accidentally – due to manual configuration errors).
  • To provide a central mechanism to keep track of all the assigned IP addresses in the network.
  • To automatically reclaim unused IP addresses and re-assign them to other devices in the network.
  • To make central configuration/ topology changes in the network easier.
  • To dynamically allocate IP addresses to portable clients / wireless devices that keep moving from one place to another in the network.

IP Address Range: Certain IP addresses (and IP address ranges) can be excluded from being allocated to clients by the DHCP Server – These might be static IP addresses that are assigned to the servers, for example. It is also possible to assign the same IP addresses to certain network devices repeatedly, by the DHCP Server itself. The range of IP addresses that can be allocated by the DHCP Server needs to be specified by the administrator.

Redundancy: When the lease period expires, the client will normally contact the DHCP Server to renew the lease. If the server is up, it will be renewed. But if the server is not functioning, the client will broadcast the renew request to the network hoping that some other DHCP server in the network can renew the lease, if it is configured to do so. So, some level of redundancy is built into DHCP.

Security: DHCP does not provide much of security as there is no authentication process for either the DHCP client or the DHCP server. So, its possible for rogue servers to claim as genuine DHCP servers and rogue clients to overwhelm the DHCP server with too many false IP address requests.

DDI – DNS DHCP & IP Address Management is catching up with large distributed enterprises for centralized management/ security/ redundancy of these critical functions and you might want to read about DDI from here.

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One Comment

  • vempati


    Can you please elobarate DHCP in a Multiple subnet & Multiple VLAN scenario. How Network switches relay the request and how DHCP understands and assigns address to that particular VLAN where the request from