CompTIA Network+ Certification Exam Training:
DHCP Discover, Offer, Request, and Acknowledgement Packets Illustrated And Explained
By Chris Bryant, CCIE #12933
When it comes to assigning an IP address to all the PCs on our network, along with their network mask, DNS server location, and more, we've got two choices on how to do it:
- Go to every workstation and configure the workstation with a static IP address, network mask, etc.
- Go to every workstation and configure them all to get an address dynamically
What we have here is the classic "static vs. dynamic" argument, and I don't want you to think I'm lazy, but I'll take the dynamic way of doing things almost every single time.
You may wonder why it matters, since both methods involve visiting each workstation. You're right about that, and even though it's a lot quicker to configure a workstation to get its IP address and mask from a DHCP server than it is to configure the entire IP address and mask statically, the real benefits come in when the network changes.
And take it from me - your network will change. You'll remove hosts, you'll add hosts, and if the previous network manager didn't plan for future growth, the day may come when you've got to change the IP numbering scheme for your network. The choice that was made originally between static configuration and DHCP will then determine how easy the change will be.
- If the network was statically configured, you will now have to go to every single workstation and change their IP addressing to the new scheme.
- If the network is using DHCP, you simply change the networking scheme on the DHCP server and allow the workstations to get their new addresses dynamically.
Believe me, I've performed IP address changes in both fashions, and I'll take DHCP every time!
When hosts receive an IP address from DHCP, it does not belong to that host permanently. The address is actually leased from a DHCP server. Let's walk through the DHCP process from the host's point of view.
First, the host (DHCP Client) boots up, and sends a DHCP Discover packet onto the network. The host does this in order to "Discover" a DHCP server or servers. This Discover packet is a broadcast.
Every DHCP Server that hears this broadcast will reply with a DHCP Offer, and it's an IP address and mask that is being offered. The DHCP Offer also includes a subnet mask, the IP address of the DHCP Server sending the response, and how long the host can keep this address (the DHCP lease duration).
If multiple DHCP Servers happen to hear this broadcast, they will each offer an IP address.
The DHCP Client will accept the first offer it receives. It does so by sending another broadcast, a DHCP Request packet.
The DHCP Server that made the offer that's being accepted will now send a DHCP Acknowledgement, or ack, which contains the rest of the information the host needs to function, including the location of a DNS server. The DHCP Server that made the offer that was not accepted will return the offered IP address to its range of assignable addresses, its address pool.
I made several mentions in this section about these DHCP packets being broadcasts. Do you remember which network connectivity device does not forward broadcasts?
That's right, it's our old friend, the router! I didn't label the router in this diagram and will not do so in other sections, because you need to recognize that symbol as a router - it's a very common symbol in network diagrams and certification exams!
If that PC sends a DHCP Discover packet, the DHCP Server cannot receive it, because routers do not forward broadcasts. We've got a problem there, but for every problem there is a solution... and we'll discuss this solution and other DHCP details in a future Network+ certification exam tutorial!
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To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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