DHCP & DNS

DHCP (Dynamic Host Configuration Protocol)

DHCP is network protocol that is used for assigning various network parameter to a device or a system. This greatly simplify administration of network, since there is no need to assign manually network parameters for every devices.

DHCP is client-server protocol. A client is a network device that is configure for using DHCP request network parameter from DHCP server. DHCP server maintain a pool available IP addresses and assign one to them to host. A DHCP server also provide some other parameters, like that:

·         subnet mask

·         default gateway

·         domain name

·         DNS server

 

Cisco router can be configuring as both DHCP clients and DHCP servers.

DHCP process explained:

DHCP client goes through the four step:

1: A DHCP clients send a broadcast packet (DHCP Discover packet) to discover DHCP server on LAN.

2: The DHCP server receive the DHCP Discover packet from client and respond with DHCP Offer packet to the client, and offering IP address information.

3: If the client receive the DHCP Offer packet from many DHCP server, the first DHCP Offer packet is accept. The client respond by broadcasting a DHCP Request packets to the server, and requesting the network parameters from the server that respond first.

4: The DHCP server approve lease with DHCP Acknowledgement packets. The packet includes lease duration and other configuration.

NOTE
DHCP use a well-known UDP ports number 67 for DHCP server, and UDP port number 68 for client.

 

DNS (Domain Name System)

DNS is a network protocol used for translating hostname into IP address. DNS is not requiring for establishing a network connection, but it is more user friendly for users than numeric addressing scheme. For example – you can access the Google by typing 216.58.207.206, but it is more easier just by typing www.google.com!

For using DNS, you have a DNS server configure to handle the translation process. A DNS has special-purpose application installed. This application maintains table of dynamic or static hostnames to IP addresses mapping. When user requests some network resource using with hostname, (Like, by typing www.google.com in a any browser), a DNS request is sent to DNS server asking for IP address of the hostname. Then DNS server replies with IP address. The user browser can now use that IP address to access www.google.com page.

The below example explain the concept of DNS:

Suppose that the DNS Client want to commune with server named Server 1. Since DNS Client does not know the IP address of Server 1, it sends DNS Request to DNS Server, asking for Server 1 IP address. The DNS Server replies with IP address of Server 1 (DNS Reply).

The below example shows a sample DNS record, which taken from a DNS server:

Here you see that host with the hostname APP1 is using the IP address of 10.0.0.3.

NOTE
DNS use well-known UDP port is 53.