Computer Networks - DHCP

1. Dynamic Host
Configuration Protocol

2. DHCP
 A host need an IP Address and some other piece of information (subnet mask,
gateway address, etc…) to send the packets.
 Normally this information’s are provided by the system administrator and
configure in manually.
 But for a large network it is complex and error prone. So automatic
configuration method is used.
 Automatic configuration is provided by DHCP.
 It needs a DHCP server to provide the information dynamically.
 DHCP Server – It is a central repository for host configuration details.

3. DHCP
Steps,
 A newly booted or attached host sends a “DHCP DISCOVER”
message to special IP Address 255.255.255.255 (Broadcast Address).
 DHCP relay receives this request and unicast that request to the
DHCP server.
 DHCP relay – It contains the IP address of DHCP Server.
 DHCP server sends back the response to the requesting client.

4. DHCP
 DHCP dynamically assigns IP address to hosts. But host cannot keep
addresses indefinitely.
 Thus DHCP allows addresses to be “leased” for some period of time.
Once the lease expires, client needs to renew the IP Address.
Advantages,
 Scaling of network management
 Improves manageability of a network.

5. Unicast
Broadcast
Host
DHCP
Relay Other Networks DHCP
Server

7. DHCP
 Operation Code (8 bit) – specifies whether the message is request
(1) / response (2).
 Hardware Type (8 bit)–Specifies the type of hardware used for local
network (value 1 – Ethernet).
 Hardware Address Length (8 bit)–Specifies the length of the
hardware address.

8. DHCP
 Hops (8 bit) – Used to control the DHCP request message by DHCP
relay.
 Transaction Identifier (32 bit) – XID - A 32-bit identification field
generated by the client, to allow it to match up the request with
replies received from DHCP servers.

9. DHCP
 Seconds (16 bit) -number of seconds elapsed since a client began an
attempt to acquire or renew a lease.
 Flag (16 bit) –B bit is set to 1 to represent the broadcast message.
Other bits are set to Zero.
 Client IP Address - The client puts its own current IP address in this
field if and only if it has a valid IP address while in the renewing state.

10. DHCP
 Your IP Address - The IP address that the server is assigning to the
client.
 Server IP Address –Address of the DHCP server.
 CHAddr –Client Hardware Address - The hardware address of the
client, which is used for identification and communication.
 SName –Server Name – Name of the DHCP server, provided by the
server during the response.