what is dynamic host configuration protocol(DHCP)?

what is dynamic host configuration protocol?. If You Find The Answer Of This Question Then This Article Is Definitely For You. In This Article, We Will Discuss the DHCP (dynamic host configuration protocol). So Without Wasting Time Let’s Know About The CSS.

what is dynamic host configuration protocol (DHCP) 

This protocol is used to assign IP addresses to hosts or workstations on the network. Usually a DHCP server on the network performs this function. Basically it leases out address for specific times to the various hosts.

If a host does not use a given address for some period of time, that IP address can then be assigned to another machine by the DHCP server. When assignments are made or changed, the DHCP server must update the information in the DNS server.

As with BOOTP, DHCP uses the machines or NIC Ethernet (MAC) or hardware address to determine IP address assignments. The DHCP protocol is built on BOOTP and replaces BOOTP. DHCP extends the vendor-specific area in BOOTP to 312 bytes from 64. RFC 1541 defines DHCP.


DHCP RFCS are 1533, 1534, 1541, and 1542. Sent from DHCP server:

  •   IP address. 
  • Netmask. 
  •  Default Gateway address. 
  • DNS server address (es). 
  • NetBIOS Name server (NBNS) address(es). 
  • Lease period in hours. 
  • IP address of DHCP server.

DHCP Lease Stages

 1. Lease Request :
The client sends a broadcast requesting an IP address.

 2. Lease Offer :
The server sends the above information and marks the offered address as unavailable. The message sent is a DHCPOFFER broadcast message.

 3. Lease Acceptance :
The first offer received by the client is accepted. The acceptance is sent from the client as a broadcast (DHCPREQUEST message) including the IP address of the DNS server that sent the accepted offer. Other DHCP servers retract their offers and mark the offered address as available and the accepted address as unavailable. 

4. Server lease acknowledgement :
The server sends a DHCPACK or a DHCPNACK if an unavailable address was requested. 

DHCP discover message 

The initial broadcast sent by the client to obtain a DHCP lease. It contains the client MAC address and computer name. This is a broadcast using as the destination address and as the source address. The request is sent, then the client waits one second for an offer.

The request is repeated at 9, 13 and 16 second intervals with additional 0 to 1000 milliseconds of randomness. The attempt is repeated every 5 minutes thereafter.

 The client uses its own port 68 as the source port with port 67 as the destination port on the server to send the request to the server. The server uses its own port 67 as the source port with port 68 as the destination port on the client to reply to the client.

Therefore, the server is listening and sending on its own port 67 and the client is listening and sending on its own port 68. This can be confusing when you consider which way the message is going.
To be clear on this, I quote RFC 1531 which states “DHCP messages from a client to a server are sent to the DHCP server’ port (67), and DHCP messages from a server to a client are sent to the ‘DHCP client’ port (68)”. 

DHCP Lease Renewal

 After 50% of the lease time has passed, the client will attempt to renew the lease with the original DHCP server that it obtained the lease from using a DHCPREQUEST message.
Any time the client boots and the lease is 50% or more passed, the client will attempt to renew the lease. At 87.5% of the lease completion, the client will attempt to contact any DHCP server for a new lease.

If the lease expires, the client will send a request as in the initial boot when the client had no IP address. If this fails, the client TCP/IP stack will cease functioning. 

  • DHCP Scope and Subnets 

One DHCP scope is required for each subnet.

  •  DHCP Relay Agents 

DHCP relay agents may be placed in two places: 

  1. Routers 
  2.  Subnets that don’t have a DHCP server to forward DHCP requests. 
  • Client reservation

client is used to be sure a computer gets the same IP address all the time Therefore, since DHCP IP address assignments use MAC addresses to control assignments, the following are required for elient reservation: 

  1. MAC (hardware) address 
  2. IP address. 
  • Exclusion Range 

Exclusion range is used to reserve a bank of IP addresses so computers with static IP addresses, such as servers may use the assigned addresses in this range. These addresses are not assigned by the DHCP server. 

This demonstrates that the IP addresses are based on lease times to the various clients. If they are not used within the period of their lease time by the client, those IP addresses are freed up for use by other clients.


There are many network devices used in networking. They are explained below: 

  • Network Repeater 

A repeater connects two segments of your network cable. It retimes and regenerates the signals to proper amplitudes and sends them to the other segments.

When talking about, Ethernet topology, you are probably talking about using a hub as a repeater. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network communication when there are several repeaters in a row.

Many network architectures limit the number of repeaters that can be used in a row. Repeaters work only at the physical layer of the OSI network model. 2.8.2. Bridge A bridge reads the outermost section of data on the data packet, to tell where the message is going.

It reduces the traffic on other network segments, since it does not send all packets. Bridges can be programmed to reject packets from particular networks.

Bridging occurs at the data link layer of the OSI model, which means the bridge cannot read IP addresses, but only the outermost hardware address of the packet.

In our case the bridge can read the Ethernet data which gives the hardware address of the destination address, not the IP address. Bridges forward all broadcast messages.

Only a special bridge called a translation bridge will allow two networks of different architectures to be connected. Bridges do not normally allow connection of networks with different architectures. The hardware address is also called the MAC (media access control) address.

To determine the network segment a MAC address belongs to, bridges use one of :
* Transparent Bridging:
They build a table of addresses (bridging table) as they receive packets. If the address is not in the bridging table, the packet is forwarded to all segments other than the one it came from. This type of bridge is used on Ethernet networks.
*Source route bridging:
The source computer provides path information inside the packet. This is used on the Token Ring networks.
References –
DHCP – help.ubnt
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