Dynamic Host Configuration Protocol | Networking Services

Dynamic Host Configuration Protocol (DHCP) is a network protocol used to configure network devices to enable them to communicate on an IP network. A DHCP client uses the DHCP protocol to acquire configuration information, such as an IP address, a default route and one or more DNS server addresses from a DHCP server. The DHCP client then uses this information to configure its host. Once the configuration process is complete, the host is able to communicate with other computers on the network and/or access the internet.

DHCP is widely used in enterprise networks and by ISP’s as they usually serve a large number of computers. On large networks, using DHCP servers helps administrators to automate the procedure of assigning IP addresses to individual computers on the network.

DHCP infrastructure consists of:

  • DHCP Servers
  • DHCP Clients

DHCP server maintains a database of IP addresses and configuration information. When it receives a request from a client, the DHCP server allocates an IP address from a given range and sends the configuration information to the client. DHCP servers are preconfigured with a range of IP address and additional network configuration information by the administrator. Like other TCP/IP services, DHCP uses port numbers 67 & 68.

DHCP Process

DHCP follows a basic process to automatically configure a DHCP client, widely referred to as DORA:

  1. DHCP client sends a DHCP Discover message.
  2. DHCP server(s) responds with a DHCP Offer message.
  3. DHCP client selects an IP address offered and sends a DHCP Request message to request use of this configuration.
  1. DHCP server assigns the IP address and sends DHCP Acknowledge message to the client.

The DHCP Request message identifies the server whose offer the DHCP client selected. The other DHCP servers which had sent offers, place their offered IPv4 addresses back into the available pool of addresses.

Auto-IP

Computers configured as DHCP clients receive an IP addresses from a DHCP server. If the DHCP server is unavailable or the DHCP has exhausted all its IP address, DHCP clients will never receive an IP address. This could lead to communication problems between computers on a network.

To addresses such issues, Auto-IP comes into effect. In this process, computers or hosts select a random IP address within a reserved range (built-in within the operating system) in order to communicate with other computers within that network. To ensure there are no IP conflicts in this automated process, hosts use ARP probes to determine if the address is already in use in the network; if there is a conflict, another random address within the range is selected. The IP address is used only when there are no replies to the ARP probe, indicating availability of the address.

Internet Engineering Task Force has reserved the address block 169.254.1.0 through 169.254.254.255 for Auto-IP reserve range in IPv4. Auto-IP is a feature found on most recent operating systems starting with the release of Microsoft Windows XP. Recent MAC OS and linux distributions also have support for Auto-IP.

File and Print Sharing

Once you have setup the computer to work in a network, you can share files and printers among other computers a network. You can use a variety of protocols for file and print sharing depending on the level of support that is available within the operating system. Likewise, you can use either static or dynamic IP addresses as well and share or access files across different computers in a network.

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