CompTIA Network + Certification Exam Tutorial:

DHCP And RARP

By Chris Bryant, CCIE #12933

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.

Earlier in this Network+ exam tutorial, I mentioned that there may come a time when you need to change IP assignments on your DHCP Server.  But what about the IP addresses that the hosts already have?  You can force the host to release its current DHCP-assigned IP address by going to the command prompt and entering "ipconfig /release" (without the quotation marks).  To then have the PC request a new IP address, enter "ipconfig /renew".

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com