DHCP (Dynamic Host Configuration Protocol)

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DHCP stands for Dynamic Host Configuration Protocol. It used to provide network configuration information's to clients automatically.For an example, if a device comes into the network, a server send the all the network parameter to he device, rather than configure IP address,subnet mask and DNS IP manually or statically through network administrator,this is what call as DHCP.

It uses UDP(User Datagram Protocol) DHCP server used port number 67 and DHCP client used port number 68.It reduces the load for Network Administrator and easy to assign IP addresses for large networks.

DHCP uses DORA process :
    • D - Discover
    • O - Offer
    • R - Request
    • A - Acknowledgement

                                                                                         Client(Port 68)                                               Server(Port 67)


Step 1 : If a device comes into the network it sends the Discover messege to  DHCP server.
              (Before it send the Discover message to DHCP server, Client send the broadcast message for find the DHCP server's IP address.)

Step 2 : DHCP server Offer the IP address which is available in IP pool.
              (To make sure the Offer IP address is not using any device DHCP server send the ACK message to all devices which are into the Network pool, then DHCP sever will Offer new IP address to the device)

Step 3 : After client received the Offer message from DHCP server, It Request to DHCP server to map that IP address with that device.

Step 4 : Then DHCP server map that IPaddress with that device and send Acknowledge message to the device.

 After got Acknowledgement the device get new IP address , Network mask, DNS informations and Gateway address

How to assign IP address through DHCP with cisco router

Step 1 : We have to tell to the server what are he IP addreses not to assaign to device
 CISCO(config)#ip dhcp excluded-address start-ip-address end-ip-address
for example:
 CISCO(config)#ip dhcp excluded-address 10.5.5.1 10.5.5.2

Step 2 : create a DHCP pool with a name(you can use any name).
 CISCO(config)# ip dhcp pool pool_name

Step 3 : Into the DHCP pool we have to define IP range with subnet mask.
 CISCO(dhcp-config)#network 10.5.5.0 /27

Step 4 : Then define the DNS server 
 CISCO(dhcp-config)#dns-server 10.5.5.0



Step 5 :To verify what are the IP addresses assign for devices you can use this command
 CISCO#show ip dhcp binding
it will show all assigned IP addresses with associate MAC addresses


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