>> Our next network service we'll take a look at is DHCP,
Dynamic Host Configuration Protocol.
This allows a client to come onto the network send out
a query and receive an IP address automatically.
Without DHCP, we would have to go to
each system and statically configure an IP address.
This automates our process and makes life much easier.
You can see on the screen,
a shot from windows where we've got the DHCP utility.
One of the most important things we have to think
about doing is setting up a scope.
A scope is a range of
IP addresses the DHCP server can issue.
I will have a scope between 1011100,
and 1011200 for instance.
Then DHCP would be able to issue
IP addresses anywhere in that range.
When I say DHCP will issue those addresses,
it doesn't give a client
IP addresses forever, ain't nothing free.
What DHCP does
>> is it leases an IP address to the client.
>> The typical lease is eight days.
It will release an address to a client for eight days.
If that client wants to renew their lease,
they can contact DHCP after four days and ask,
"Hey, can I continue my lease?"
And if DHCP is accessible,
it says, "Sure."
If not, the client will try again and again.
If it can't contact the DHCP server,
the entire process just starts over.
We'll talk about what that leasing process
is in just a second.
You can also reserve IP addresses for specific servers.
When you reserve an IP address,
let's say I have a file server and I'd like
that file server to always have the same IP address,
I can create a reservation for that file server,
enter its MAC address.
That way when that file server comes online
to get an IP address from DHCP,
based on its MAC,
it will be given that specific reserved IP address.
I can also exclude IP addresses from the range.
Maybe I'm going to give out IP addresses
>> from 1011100 to 1011200 but 101150 is
>> a print server that I want to assign manually.
I can just exclude from that range and
101150 won't be given to any other device.
I'll have to manually configure the print server.
With reservations,
a specific IP address is given to a specific host.
With exclusions,
the IP address is removed from the range,
and that requires the administrator to
manually assign that address.
Sometimes exclusions are better.
If you have really critical servers,
it's best to just statically assign an IP address.
Some services require that you do,
but also if you have
a really critical server and you think about what would
happen if DHCP is down
and I couldn't access that critical server,
that could really be a problem.
When you have those really important
servers on the network,
it's probably just better to go ahead and give them
a static address and
>> exclude the address from that range.
>> I mentioned the leasing process just a few minutes ago.
Let's go ahead and look at this.
You can remember the DHCP lease process through DORA;
Discover, Offer, Request, Acknowledge.
The way the discover process
works is when a client comes online,
it sends out a broadcast message that basically says,
"Hey, is anybody out there?
DHCP server?"
Every DHCP server that
here's the query, responds and says,
"I'm a DHCP server and here's an IP address for you."
The client is going to request
the first IP address that it received as an offer.
Then that DHCP server is
going to come back and acknowledge the client has
been offered an IP address and it's
going to remove the IP address from its scope.
That's the DORA process.
There are a couple little things to note here.
First of all, to start
the discover message is a broadcast.
There are some devices that don't allow broadcasts
to pass, specifically routers.
We'll talk in a later chapter about what routers
are and some of the peculiarities of them.
But one of the things a router does is lock broadcasts.
There are some broadcasts you don't want
to go through your entire network.
You might want some broadcasts
limited to certain segments,
a router does that for us.
But if I'm trying to get an IP address
on the other side of a router,
then my broadcast is going to be blocked.
There are a couple of things we can do about this.
There's something called a DHCP relay agent,
which is a kind of service you install on
the router that will afford those DHCP requests.
There are also certain routers referred to as
RFC 1542 compliant routers.
You can also hear them referred to as boot P routers.
These will also forward
those discover messages from clients.
The next service we look at is called
IPAM, IP address management.
We may be in an environment that has
multiple locations throughout the world.
We may have thousands and thousands of hosts.
When you start to work with a very large organization,
becomes very challenging to
keep up with all your network segments,
the IP addresses, and
any sort of naming resolution issues.
There are a series of software tools that will assist
you with determining what IP addresses are in use,
whether or not they're being fully utilized,
and any sort of issues that might
have with your DHCP server scope.
You could also even use it with
incident response because it's
able to detect the IP addresses that are being used.
That's going to bring us to the end of the services.
We looked at DNS,
DNS for name resolution,
and determining where services are.
We looked at DHCP for automatic IP address assignment.
Then we looked at IPAM as a means of managing
a more complex environment and keeping
track of IP address scopes and names as well.