Router As DHCP Server

We love technologies that make our work easy ,one such technologies is DHCP when the network gets bigger and your network admin becomes unreachable to every device on the network he calls one of his friend in deed yes you guessed it right it’s his friend DHCP. Well someone’s friend can be your friend but you have to know him first or you just have to know how useful he can be to you in tough situations or if just want a Cisco Certification .Ok I guess much of going in circles let’s get to the point following lines are dedicated to DHCP working in the network it will help you in Cisco CCNA Training.

Dynamic Host Configuration Protocol (DHCP)/Bootstrap

Protocol (BootP)

Dynamic Host Configuration Protocol (DHCP) assigns IP addresses to hosts. It allows for

easier administration and works well in small to very large network environments. Many

types of hardware can be used as a DHCP server, including a Cisco router.

DHCP differs from BootP in that BootP assigns an IP address to a host but the host’s

hardware address must be entered manually in a BootP table. You can think of DHCP as

a dynamic BootP. But remember that BootP is also used to send an operating system that

a host can boot from. DHCP can’t do that.

But there’s still a lot of information a DHCP server can provide to a host when the host

is requesting an IP address from the DHCP server. Networking Certification in Delhi is quite helpful to secure your job in IT field, you know fundamentals are never going to change. Here’s a list of the most common types

of information a DHCP server can provide:

uu IP address

uu Subnet mask

uu Domain name

uu Default gateway (routers)

uu DNS server address

uu WINS server address

A client that sends out a DHCP Discover message in order to receive an IP address sends

out a broadcast at both layer 2 and layer 3.

uu The layer 2 broadcast is all Fs in hex, which looks like this: ff:ff:ff:ff:ff:ff.

uu The layer 3 broadcast is, which means all networks and all hosts.

DHCP is connectionless, which means it uses User Datagram Protocol (UDP) at the

Transport layer, also known as the Host-to-Host layer, which we’ll talk about later.

Seeing is believing, so here’s an example of output from my analyzer showing the layer 2

and layer 3 broadcasts:

Ethernet II, Src: (00:0b:db:99:d3:5e),Dst: Broadcast(ff:ff:ff:ff:ff:ff)

Internet Protocol, Src: (,Dst:

The Data Link and Network layers are both sending out “all hands” broadcasts saying,

“Help—I don’t know my IP address!”

DHCP client four-step process

This is the four-step process a client takes to receive an IP address from a DHCP server:

  1. The DHCP client broadcasts a DHCP Discover message looking for a DHCP server

(Port 67).

  1. The DHCP server that received the DHCP Discover message sends a layer 2 unicast

DHCP Offer message back to the host.

  1. The client then broadcasts to the server a DHCP Request message asking for the

offered IP address and possibly other information.

  1. The server finalizes the exchange with a unicast DHCP Acknowledgment message.

DHCP Conflicts

A DHCP address conflict occurs when two hosts use the same IP address. This sounds bad,

and it is! We’ll never even have to discuss this problem once we get to the chapter on IPv6!

During IP address assignment, a DHCP server checks for conflicts using the Ping program

to test the availability of the address before it’s assigned from the pool. If no host replies, then

the DHCP server assumes that the IP address is not already allocated. This helps the server

know that it’s providing a good address, but what about the host? To provide extra protection

against that terrible IP conflict issue, the host can broadcast for its own address!

A host uses something called a gratuitous ARP to help avoid a possible duplicate address.

The DHCP client sends an ARP broadcast out on the local LAN or VLAN using its newly

assigned address to solve conflicts before they occur.

So, if an IP address conflict is detected, the address is removed from the DHCP pool

(scope), and it’s really important to remember that the address will not be assigned to a

host until the administrator resolves the conflict by hand!

Guys now you how what DHCP server is and how it works in the network there are other various fun protocols that can help you achieve what you want Networking Training in Delhi is definitely going to help you in long run.

Automatic Private IP Addressing (APIPA)

Okay, so what happens if you have a few hosts connected together with a switch or hub

and you don’t have a DHCP server? You can add IP information by hand, known as static

IP addressing, but later Windows operating systems provide a feature called Automatic

Private IP Addressing (APIPA). With APIPA, clients can automatically self-configure an

IP address and subnet mask—basic IP information that hosts use to communicate—when

a DHCP server isn’t available. The IP address range for APIPA is through The client also configures itself with a default Class B subnet mask of

But when you’re in your corporate network working and you have a DHCP server running,

and your host shows that it’s using this IP address range, it means that either your

DHCP client on the host is not working or the server is down or can’t be reached due to

some network issue. Believe me—I don’t know anyone who’s seen a host in this address

range and has been happy about it!

Now, let’s take a look at the Transport layer, or what the DoD calls the Host-to-Host


Configuration of DHCP server on router

To configure a DHCP server for your hosts, you need the following information at


Network and mask for each LAN Network ID, also called a scope. All addresses in a

subnet can be leased to hosts by default.

Reserved/excluded addresses Reserved addresses for printers, servers, routers, etc.

These addresses will not be handed out to hosts. I usually reserve the first address of

each subnet for the router, but you don’t have to do this.

Default router This is the router’s address for each LAN.

DNS address A list of DNS server addresses provided to hosts so they can resolve names.

Here are your configuration steps:

  1. Exclude the addresses you want to reserve. The reason you do this step first is

because as soon as you set a network ID, the DHCP service will start responding

to client requests.4

  1. Create your pool for each LAN using a unique name.
  2. Choose the network ID and subnet mask for the DHCP pool that the server will use to

provide addresses to hosts.

  1. Add the address used for the default gateway of the subnet.
  2. Provide the DNS server address(es).
  3. If you don’t want to use the default lease time of 24 hours, you need to set the lease

time in days, hours, and minutes.

Verifying DHCP on Cisco IOS

There are some really useful verification commands to use on a Cisco IOS device for monitoring

and verifying a DHCP service. You’ll get to see the output for these commands when

I build the network in Chapter 8, “IP Routing,” and add DHCP to the two remote LANs. I

just want you to begin getting familiar with them, so here’s a list of four very important ones

and what they do:

show ip dhcp binding Lists state information about each IP address currently leased to

a client.

show ip dhcp pool [poolname] Lists the configured range of IP addresses, plus statistics

for the number of currently leased addresses and the high watermark for leases from

each pool.

show ip dhcp server statistics Lists DHCP server statistics—a lot of them!

career in networking you should definitely do Networking Security Training and later even get CCNA Certification in Delhi.


Codec Networks provides IT Trainings from EC Council CEH ECSA, LPT, CHFI, Network Security, Penetration Testing, ISACA, ISC2, PECB ISO 27001LA LI, Cisco Networking CCNA CCNP, Linux Administration RHCE, Prog Languages JAVA, Advanced Java, android development. We also offer B2B Industry Solutions and Services in IT | Information|Cyber Security in Delhi NCR India.

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