Understanding DHCP – Dynamic Host Configuration Protocol and APIPA – Automatic Private IP Addressing
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In this section, we’re going to be talking about how to get IP addresses onto client machines, or workstations using DHCP – Dynamic Host Configuration Protocol and APIPA – Automatic Private IP Addressing.
Let’s take a look at these options.
In Dynamic Host Configuration Protocol (DHCP), the focus is the word dynamic. In fact, think about there was a day when there was no DHCP. For example, let’s say the boss comes in Friday afternoon and says, “I’m going to need you to come in over the weekend and go to every single workstation in the entire company and type IP addresses, subnet masks, default gateways and DNS entries and you need to do that all manually. That’d be great!” You’re probably thinking, “No, that wouldn’t be great. That would be a poor way to spend a weekend.”
In this example, DHCP will dynamically take care of that for you. A DHCP server, will take care of the responsibility for you. It will hand out addresses to clients who request an address, keep track to make sure it doesn’t give out duplicate addresses. As addresses are unused, DHCP will scavenge them back for reuse. In DHCP, it is automatically or dynamically done for you.
Automatic Private IP Addressing (APIPA), is Microsoft’s contribution to the networking field. You’ll probably recognize it if you’re ever looking at a workstation. You see and address of 169.254.x.x. We usually look at that as network admins and say, “Something’s not working right?” Not necessarily. Microsoft built it so that if the machine is configured as a DHCP server but the DHCP server is unavailable. Either doesn’t answer. For some reason, this host can’t get to the DHCP server.
It will try, until eventually, it’ll notify you that the DHCP server can’t be reached so it will give you an address that’s is an Automatic Private IP Addressing (APIPA) or another term for it is Link‑Local.
In IPv6, Link‑Local addresses are used all over the place, much more important than an IPv4.
DHCP Configuration:
The actual configuration, if you use the shortcut is ncpa.cpl which stands for Network Control Panel Applet, it will get you quickly to the screen where you can right‑click, hit properties, and achieve this view here.
In a command prompt, we’ll type ncpa.cpl and hit enter.
Look how quickly we get there.
Next we’ll right‑click and select click properties,
This takes us to the property sheet. It was not a big deal to get there at all.
You notice the below image is of IPv4 properties.
IPv4 properties is here, and then there’s the button.
Click Properties to see how it’s been configured.
This particular machine obtained an IP address automatically.
Obtained a DNS Server Automatically. That’s the default setting and the way the network admin would like it to be because everything is just handled for you. That’s how to configure it.
Again, this particular lesson is simply client‑side. Obviously, we’re not talking about how to configure a DHCP server in iOS or in Linux, whatever. This is client‑side on a Windows box if I want DHCP.
APIPA Configuration:
What happens if DHCP does not give the address that this client has been told to ask for? That’s the alternate configuration.
Notice the tab in the propertied window that provides alternate configurations. One option is Automatic private IP address.
You’ll notice it’s not the only option, but most of the time, that’s what people pick.
You could configure this that if it’s unable to get an address, If we wanted to have xyz address. Then you run into where you have to make sure that the human beings are keeping track so that you don’t have duplicates.
These are your options in IPv4. DHCP is your best friend when you want this to be handled automatically. If you see 169.254 addresses, you’re thinking in IPv4, there’s an issue.
Now you’ve seen the way you want your network to be configured. DHCP, Dynamic Host Configuration Protocol a huge time saver.
Until next time….
Mark Jacob
Cisco and CompTIA Network + Instructor – Interface Technical Training
Phoenix, AZ
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