What are subnets?

At Miso, we recently launched an integration with AT&T that allows the Miso iPhone app to connect with your TV. Have you ever thought that browsing hundreds of cable channels on your TV was a painful process? Our vision with Miso is that you can turn on your favorite show, see what your friends are watching, or browse additional information about a show, all from your phone.

To make this happen, Miso connects with the AT&T receiver over wifi, and we’re learning from our early adopters that home networks are more complicated than they used to be. Wifi routers are definitely the norm, and it’s not uncommon to have multiple wifi routers cover the living room, the master bedroom, and the guest bedroom upstairs. If you’re plugging in a wifi router that you bought from Best Buy, you’re probably creating a new subnet, which is something you almost never need to care about.

As it turns out, Miso’s integration with AT&T and DIRECTV both assume that a home only has one subnet, which is not always true. This has caused some support issues and prompted some people within Miso to ask, what is a subnet? Good question! As a programmer, I have some basic knowledge of network engineering, but answering this question prompted me to dig a bit deeper.

What is a subnet?

Subnets and IP addresses go hand in hand. Together, they are the building blocks for how computers find each other on a network. For instance, every time you visit a website, you first have to find the computer on the internet that hosts that website.

Addressing on the internet works a lot like mailing addresses for your apartment. The zip code for my apartment is 94105; if you were to Google that, you could see that I live within a certain region of San Francisco. Subnets are like zipcodes – they divide up the internet world into small regions.

The subnet is actually part of the IP address. You’ve probably seen an IP address for a computer. My IP address at this coffee shop is 192.168.5.199. The first three numbers (192.168.5) is the subnet (or network address) and the last number (199) is my laptop’s address (host address). The guy sitting at the next table probably has an address like 192.168.5.198. Because we’re on the same subnet, we could do things like share playlists with iTunes or exchange files on a shared folder. The three numbers that form the subnet are somewhat random, in the same way that zipcodes are somewhat random.

For nerds only: the tricky part is that which part of the IP address is the subnet actually varies, and it determines how large the subnet is. The mechanisms for dividing up address space has evolved over the years, from classful addressing to classless addressing. We’re at the point where we’re out of IPv4 addresses, thus the creation of IPv6 which has many more available addresses.

So, great, subnets are like zipcodes. They group computers together in some logical way. Laptops in the same cafe are probably on the same subnet and can do stuff like share iTunes playlists, and laptops that are in different cafes probably can’t. Yet, there must be some way to communicate between subnets. The websites that I visit every day are hosted on computers that are all on different subnets.

How does information travel between subnets?

Sending information across subnets is a lot like sending mail across the country. After writing the address on the envelope, I deliver the letter to my local post office. My local post office looks at the zipcode and begins the process of routing the letter through hubs to bring the letter in the general vicinity of the destination address. As the letter approaches the destination address, it will be given to another local post office, which ultimately puts it on a mail truck that goes to the actual address.

In the same way, let’s say that I want to visit google.com. From my laptop, the address for google.com is 74.125.224.114. My laptop has no idea where that address is, so it does the equivalent of delivering my request to the local post office, which is the wifi router of this coffee shop (you may have also heard this referred to as a gateway). The coffee shop’s wifi router also doesn’t know where that address is, so it forwards the request to the internet provider (ISP) of the coffee shop. The internet provider has a better sense for zip codes, so it starts the process of sending the packet in the right direction.

For nerds only: This process is called IP routing and there are several algorithms for doing it and they vary with IPv4 and IPv6. The algorithms differ in how chatty they are and what size network they work well with, among other things. Many algorithms are based on Dijkstra’s algorithm.

So how come I can’t share my iTunes playlist with any computer?

I can access google.com from anywhere, but I can’t access that laptop from the other coffee shop. The reason for that is that there are many more computers (and IP addresses) than there are mailing addresses, so they can’t all be public. If you work in a large business, then your department probably has an internal mail stop that’s handled by your company’s mailroom.

In the same way, internet addresses are divided up into public and private addresses. Public addresses can be reached from anywhere, but private addresses can only be accessed in local networks. Public addresses are handed out by the internet equivalent of the post office called the Internet Assigned Numbers Authority (IANA). If I want a public mailing address, I have to contact the post office. If I want a public IP address, then I have to contact my internet provider. My internet provider is a member of a Regional Internet Registry (RIR), which is a member of the IANA. It’s fairly common that one home will have one public IP address, although you can pay your ISP for more.

So, why doesn’t AT&T and DIRECTV work across multiple subnets?

Hopefully, you have a high level sense for internet addressing, subnets, and routing, although we still haven’t explained why our AT&T and DIRECTV integrations don’t work across multiple subnets yet. We need to understand one more concept – multicast.

Most network communication is two computers talking to each other directly. In some cases, you want to communicate with multiple computers at the same time. Broadcast and multicast are the two ways of doing that. In order for Miso to find the cable receiver, it makes an announcement over multicast asking if there are any cable receivers in the network. That’s the same way your computer discovers printers on your local network.

As you can imagine, multicast announcements can be pretty noisy. Imagine if my laptop made a multicast announcement and every computer in the world had to pay attention; things would get pretty chaotic. Therefore, multicast messages only go out to current subnet. If my cable receiver is on a different subnet, it won’t receive my multicast announcement.

There are a number of ways of overcoming this problem which include adjusting multicast TTL levels, router port forwarding, or configuring routers as bridges. TTL (Time-to-live) tells the multicast message how many hops to take before it should give up. By default, this value is 1, which means that it will deliver the multicast messages to all computers on the local subnet only. Router port forwarding and configuring routers as bridges are other mechanisms for allowing the multicast traffic to hop across to an adjacent subnet; however, for our AT&T integration, they are less ideal because they involve users understanding how to administer their home networks.

Hopefully, you found this high level guide useful. Feedback welcome!

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