Gone But Not Forgotten: Part 2 – DHCP

In the previous article, we explained that every system on a network needs an IP address. The truth is that every system already HAS an address based on the network adapter they connect with. It’s called a MAC address, and it’s built into the network card or Wi-Fi connection of your computer, laptop, tablet, or mobile device. The problem with these addresses is that they are wildly random, hexadecimal numbers.

An example is 3C-F8-62-D5-CB-42. If that sounds crazy and you don’t believe me, just look it up. You could search for your MAC address online, or you can just open a command prompt and type ipconfig /all, and you’ll see your wired and/or wireless LAN adapter has a physical address that resembles the example I just gave you. Mobile devices, too.

On an iPhone, just go to Settings – General – About and you’ll see it next to Wi-Fi Address. These MAC addresses are what make all devices on the Internet… the millions of devices… unique. The problem? If you had to manage a network of systems with MAC addresses, you might lose your mind. The solution? A much easier system of TCP/IP addressing, which we explained in the previous article. Not only does a TCP/IP address help create networks of systems with addresses that allow for easy communication between systems on a network, but through routers you can connect one network to another (and another, and another) to create a mesh network of systems globally (ie. the Internet). The new problem? How do you address all of these systems?

It is possible to manually configure the IP addresses of your systems (as you see in the figure below). You can configure the address, subnet mask, and default gateway manually for all of the computers within your organization and back in the day (the late 90s), admins might have preferred it. But, there is an alternative. DHCP services. DHCP (dynamic host configuration protocol) allows you to establish a range of IP addresses that can be given out on a leased basis (you determine the length of the lease). In networking, we would set up a server that had DHCP services that would allow us to configure that range. In smaller environments, you can use your Wi-Fi gateway and configure the range so that all systems that connect (desktops, laptops, tablets and mobile devices) get an in-house IP address and can access the Internet through your gateway.

Funny thing here is that we called this “older” technology, but not “dead” technology. If we wanted to discuss “dead” technology we could be talking about the typewriter, the pager, the PalmPilot, but DHCP, much like TCP/IP is very much alive! Perfect example… I use a UniFi Dream Machine for my home network. UniFi offers a variety of different IT solutions that connect back to a dashboard that is as close to a Star Trek dash as you get. When configuring the system, I had to provide it a TCP/IP range of addresses for the DHCP portion of the setup. Additional configuration options offered included DHCP guarding and a list of other configuration options that all start back with the question… what is DHCP? Well… now you know.

By default, your systems are ready for an automatic IP address to be assigned. That’s why most people don’t even think about it. You walk into Starbucks and your mobile sees the Starbucks Wi-Fi connection, and you agree to use it. Behind the scenes, your device is getting an IP address from the Starbucks Wi-Fi gateway router, and that puts you on their network. Done! And because you can access the Internet, you can access your email and other M365 solutions. You can VPN into your network, if that’s configured.

Note: Most mobiles are already connected to the Internet through a data plan, and if you’re looking to connect your laptop up while on the road, you would be safer sharing out your mobile device’s connection instead of connecting through an open network like a Starbucks, hotel, or conference center, where you might be exposing your data to someone sitting on that network, looking for an opportunity to steal access or data.

There is another type of “automatic IP addressing” where your computer can receive an IP address if it doesn’t have one assigned and can’t communicate with a DHCP server. That IP addressing is called the APIPA (or Automatic Private IP Addressing). While this may seem like a “helpful” feature, if your computer has one of these IP addresses that start with 169.254.x.x, then you won’t be able to communicate with other computers or out to the Internet.

Funny story: Years ago, one of my students worked for an airline in Newark Airport and none of the computers could communicate back with the ‘mother ship’ in Houston. The admins were frustrated and asked if I would take a look. The basement of an airport in the late 90s was all cables from the terminals above leading down to a mix and match of systems and servers below. I opened a command prompt, typed ipconfig, and knew immediately that the systems weren’t communicating because they had a self-assigned IP address (169.254.x.x). The DHCP services were in Houston, and something was preventing the assignments. But now, they knew it wasn’t their fault and could kick the blame back to Houston. 

You might be thinking, “Ok, so MAC addresses are assigned at the factory level and that’s how everyone is unique, but TCP/IP addressing helps us control and manage (with things like DHCP automatic addressing) and segment (through routers) the networks we create using 32-bit binary numbers in dotted decimal formation (x.x.x.x) and I get all that… but it still doesn’t explain how when I type in an address like www.mimecast.com, it takes me to a web page, or how email or anything else works.” Here is the good news… as an M365 admin, you may not have even cared (yesterday), but now, you’re curious, which is exactly what I’d hoped these articles would do – increase that curiosity on how this all really works.