Friday, 28 February 2014

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First of all we will have a look at how dhcp work basically, what sort of messages or queries it include.

How DHCP work. 

Schema of a typical DHCP session
  • DHCP uses the same two IANA assigned ports as BOOTP: 67/udp for the server side, and 68/udp for the client side.
  • DHCP operations fall into four basic phases. These phases are IP lease request, IP lease offer, IP lease selection, and IP lease acknowledgement.
  • After the client obtained an IP address, the client may start an address resolution query to prevent IP conflicts caused by address poll overlapping of DHCP servers.
DHCP discovery
  • The client broadcasts on the local physical subnet to find available servers. Network administrators can configure a local router to forward DHCP packets to a DHCP server on a different subnet. This client-implementation creates a UDP packet with the broadcast destination of or subnet broadcast address.
  • A client can also request its last-known IP address (in the example below, If the client is still in a network where this IP is valid, the server might grant the request. Otherwise, it depends whether the server is set up as authoritative or not. An authoritative server will deny the request, making the client ask for a new IP immediately. A non-authoritative server simply ignores the request, leading to an implementation dependent time out for the client to give up on the request and ask for a new IP.

DHCP offers

  • When a DHCP server receives an IP lease request from a client, it extends an IP lease offer. This is done by reserving an IP address for the client and sending a DHCP OFFER message across the network to the client. This message contains the client's MAC address, followed by the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer.
  • The server determines the configuration, based on the client's hardware address as specified in the CHADDR field. Here the server,, specifies the IP address in the YIADDR field.
DHCP requests

  • When the client PC receives an IP lease offer, it must tell all the other DHCP servers that it has accepted an offer. To do this, the client broadcasts a DHCP REQUEST message containing the IP address of the server that made the offer. When the other DHCP servers receive this message, they withdraw any offers that they might have made to the client. 
  • They then return the address that they had reserved for the client back to the pool of valid addresses that they can offer to another computer. Any number of DHCP servers can respond to an IP lease request, but the client can only accept one offer per network interface card.
DHCP acknowledgement

  • When the DHCP server receives the DHCP REQUEST message from the client, it initiates the final phase of the configuration process. This acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the lease duration and any other configuration information that the client might have requested. At this point, the TCP/IP configuration process is complete.
  • The server acknowledges the request and sends the acknowledgement to the client. The system as a whole expects the client to configure its network interface with the supplied options.
DHCP on Packet Tracer.

  • This tutorial is about how to configure dhcp on cisco router in packet tracer. The Dynamic Host Configuration Protocol (DHCP) is a network protocol that is used to configure network devices. DHCP allows a computer to join an IP-based network without having a pre-configured IP address. DHCP is a protocol that assigns unique IP addresses to devices, then releases and renews these addresses as devices leave and re-join the network. 
  • Internet Service Providers (ISPs) usually use DHCP to allow customers to join the Internet with minimum effort. The DHCP server maintains a database of available IP addresses and configuration information. When it receives a request from a client, the DHCP server determines the network to which the DHCP client is connected, and then allocates an IP address. DHCP servers typically grant IP addresses to clients only for a limited interval.
  • Lets apply DHCP on packet tracer.First, let us make a topology with one router on which we will apply DHCP and several client PCs. More like this one,

Now, we will apply DHCP on the router.
The commands in sequence are as follows.

  • In the following command “ip dhcp pool cisco”, we are creating a pool for DHCP called cisco. cisco is the name here and we can name it whatever we want.
  • Similarly, in the command “default-router “ we are telling the DHCP about the default route to follow.Notice, after we exit from DHCP mode, we are excluding some IP addresses by applying this command “ip dhcp excluded-addresses x-x”, where x is the starting and ending IP address respectively. We are basically reserving some IPs for our use. It can be used to attach printers, or assign it to some specific users for security purposes. You can also give dns address in dhcp by using the following command.
  • dns-server
Now, open the PC.
  • Switch to ip configuration Tab.
  • Then you will see two options static and DHCP.
  • Check DHCP radio button.


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