- [Instructor] Imagine a network that has 10 PCs.
If I were to manually configure IP addresses on each,
that would take, maybe 20 minutes total,
which really isn't that bad.
Now imagine I'm in an office building
that has thousands of PCs.
Suddenly it seems a bit more daunting.
That's one of the places
that Dynamic Host Configuration Protocol,
or DHCP, really shines.
It's a means by which I can run a DHCP server,
either on a router or a server,
and as hosts join a network, they can request an IP address,
and be given one to them.
The server can then respond to this request
and allocate them an available IP address from a pool.
DHCP doesn't simply hand IP address to clients.
There's much more information that it can supply.
DHCP will supply a client with IP address,
subnet mask, router to use as default gateway,
a lease time, which is a timer showing how long
the address is valid for, and last, DNS servers.
DNS stands for Domain Name System,
which in a nutshell is a mechanism to take
a friendly ASCII name, like gregsole.com,
and translate that to an IP address.
It's impossible for humans to remember the IP addresses
of every network resource they require,
but they can easily remember DNS names for them.
In essence, there's a hierarchy of servers
out on the internet that host DNS services.
Hosts will make requests to servers, known as resolvers,
that will ultimately communicate with this hierarchy
to resolve the DNS name to an IP address,
and pass it back to the requesting host.
While it is common for PCs and the like
to acquire DNS server addresses via DHCP,
it's also possible to statically configure them on a client.
A DHCP client is fairly simple to configure
on a SISCO device.
I'll configure this router to pull an IP
via DHCP on interface gig zero one, from my network.
Config T, interface, gig zero one, IP address DHCP.
And now, no shut.
The interface should come online and pull an IP address
via DHCP within just a few seconds.
I'll verify with show IP interface, brief.
This shows a quick reference list of the router's interfaces
and their assigned IP addresses.
Notice the command shows the IP address,
and under method it shows DHCP.
If I issue a show IP route, I also see
that it has pulled a default route from the DHCP server.
DHCP is a service that requires layer two connectivity
between the requesting host and the DHCP server.
Most enterprises want to use a centralized DHCP server,
rather than having every router act as a DHCP server.
For this type of interaction, an administrator
can employ a DHCP relay on the router
that's acting as the gateway for clients.
This relay agent will take DHCP requests from the client
and forward them to a centralized server.
The server will then respond with an IP from a pool
in the client's subnet.
This too is quite easy to configure.
Taking a look at my diagram, I'll configure interface
gig zero two to act as a DHCP relay.
The interface sub command is config T,
interface gig zero two, IP.
Helper address.
And then the address of the server. 192.168.88.1.
In my network, the central DHCP server is 192.168.88.1.
So this command says send all DHCP requests
on this interface to the server at IP 192.168.88.1.
Many smaller offices, and edge networks,
will employ DHCP clients from broadband connections,
and as simple as enabling the client is,
it should be fairly simple to commit to memory.