DHCP: CONCEPT: BASIC


REFERENCE LINK

http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfdhcp.html#wp1000879


BASIC


BASIC

 

What : DHCP (DYNAMIC HOST CONFIGURATION PROTOCOL), as name suggests, dynamically provide IP address to the host

 

DHCP supports three types of mechanism, whIle providing IP

Automatic allocation—DHCP assigns a permanent IP address to a client.

Dynamic allocation—DHCP assigns an IP address to a client for a limited period of time

Manual allocation—The network administrator assigns an IP address to a client and DHCP is used simply to convey the assigned address to the client.

 

DHCP is based on the Client/ Server model

 

DHCP SERVER

Assigns and manages IP addresses from specified address pools within the router to DHCP clients.

 DHCP Server cannot satisfy a DHCP request from its own database, it can forward the request to one or more secondary DHCP Servers defined by the network administrator

 

 

DHCP SERVER/CLIENT COMMUNICATION

 

DHCPDISCOVER: search for the DHCP server

DHCPOFFER: DHCP Server offers configuration parameters (such as an IP address, a MAC address, a domain name, and a lease for the IP address) to the client

// A DHCP client may receive offers from multiple DHCP Servers and can accept any one of the offers; however, the client usually accepts the first offer it receives. Additionally, the offer from the DHCP Server is not a guarantee that the IP address will be allocated to the client //

DHCPREQUEST: DHCP client sends this to all the DHCP servers, so that all remaining DHCP servers can reclaim their IP

DHCPACK: DHCP Server confirms that the IP address has been allocated to the client

 

DHCP SERVER IP ALLOCATION PROCESS

DHCPDISCOVER packet is examined, which contains a feild, called giaddr feild

Case:

Client is not directly connected to DHCP server

Then, giaddr field= non-zero= some IP address

DHCP server matches availiable DHCP pools with the IP address present in the giaddr feild, both must belong to the same subnet

 

Client is directly connected to DHCP server

Then, giaddr field= zero

DHCP server matches availiable DHCP pools with IP address configured on the receiving interface, both must belong to the same subnet

 

DHCP RELAY AGENT

What: A DHCP relay agent is any host that forwards DHCP packets between clients and servers. Relay agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet. Relay agent forwarding is distinct from the normal forwarding of an IP router, where IP datagrams are switched between networks somewhat transparently. Relay agents receive DHCP messages and then generate a new DHCP message to send out on another interface.


 

 

 

 

 

 

 

 

 

 

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s