Network+ Certification Exam Tutorial - DHCP And RARP
By Chris Bryant
Part of passing the Network+ exam is mastering the many different protocols and
services you're introduced to in your studies, and that's especially hard when
two protocols do much the same thing! That's the case with RARP and DHCP, so
let's take a close look at these two protocols and why we use them in the first place.
Whether you've been around networks for a while or are just getting started with
your IT career, you'll quickly notice that many basic tasks in networking can be
handled either statically or dynamically. By "statically", I mean configuring
each device in question manually; by "dynamically", I mean having it done
(almost) automatically. Generally, you're going to choose the dynamic method,
and not because it's easier or quicker - it's because dynamic methods are much
more adaptable to change, and today's networks are always changing.
For example, let's take the task of assigning IP addresses. Let's say you've got
100 PCs that need an IP address. You could go to each workstation and assign the
address, subnet mask, and default gateway personally, or you could go to each
workstation and enable each workstation to use DHCP to get its IP address from
a DHCP server.
You may wonder why you'd choose DHCP instead of static addressing - after all,
each choice involves going to the workstation personally, right? At first, yes.
But what if the addressing scheme changes? What if six months from now you need
these same PCs to be assigned addresses using a totally different addressing
scheme? If you configured the PCs manually, you've got to go around to the PCs
again and change them manually, but if you are using DHCP, you only need to
change the information on the DHCP server itself to be just about done!
Now that we've discussed why you'd use DHCP, let's talk about what it is. DHCP
is the Dynamic Host Configuration Protocol, and this protocol allows us to
configure a DHCP Server that will contain the range of addresses to be assigned,
as well as the subnet mask, default gateway, DNS servers, and other information
that we want our PCs to learn when they are booted up.
When a PC configured to get its IP address dynamically comes onto the network,
it will send a DHCP broadcast packet. This packet will be answered by every DHCP
server that receives it, and the PC will use the IP address assigned to it by
the first DHCP server that responds to the original request. The PC acknowledges
the receipt of this address to all DHCP servers via another broadcast, so
addresses sent to the PC by other DHCP servers are returned to the pool of
available addresses.
This IP address does not belong to the PC forever. When the DHCP server is
configured, the length of the DHCP Lease is set. This value is the amount of
time the host devices will retain a DHCP address assigned to them by this DHCP
server. When the lease expires, a renegotiation must take place between the DHCP
client (the host device) and the server.
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