This was long ago, but not that long ago. I had an early Android phone and would use it to connect to the campus Wi-Fi. Suddenly, it stopped working.

I mean, it worked — it told me I was connected to the named network — but once on, I couldn’t do anything.

This was all I knew, and this was new behavior, and I wanted the Wi-Fi access. This was in a corner of campus with poor-to-no data coverage, so without Wi-Fi, it was a brick.

So I reported this issue.

Weeks pass.

Paraphrase: “Go away kid, you bother me.”

In discussion with coworkers, I get the idea to install general networking tools (which didn’t come and aren’t generally default on Android devices) and figure out what exactly this issue is.

Not The Problem

Let me list the things that I knew were not the problem:

• Password
• Network configuration
• My phone in general

It connected to other Wi-Fi networks and to the data networks. The phone was alright. It’s the Wi-Fi that’s wrong.

IP and Netmasks

IPv4 addresses are always four octets. They are this way because human brains like them better that way. Given an IP address of 10.10.10.100 , the binary representation would be 00001010 00001010 00001010 01100100 , and that, as expressed as an integer, would be 168430180. Octets are better because that gives us four smaller numbers, which, because of Chunking, we can remember better than nine digits. It becomes four things not nine.

But computers don’t think of them as octets. It thinks of them as binary numbers. Remember that.

and the octets for the netmask are always the following: 255, 254, 252, 248, 240, 224, 192, 128, and 0. That may seem oddly specific, but they all have a property. They’re all, in binary, a series of ones follow by a series of zeroes.

    255 11111111
    254 11111110
    252 11111100
    248 11111000
    240 11110000
    224 11100000
    192 11000000
    128 10000000
    0 00000000

A common netmask would be 255.255.255.0

This corresponds to the binary number 11111111 11111111 11111111 00000000. A netmask that intersperses zeroes and ones wouldn’t make sense, because the point is to differentiate the machines that are within the same network and the ones outside.

DHCP , the Dynamic Host Configuration Protocol , is used for networks, wired and wireless alike, to provide devices what they need to participate on the network. Among these are the netmask, an IP address for the host itself, and a default gateway , which is where the rest of the Internet is.

There are ranges that are private, that may be used by anybody but are not unique. One of those ranges is 10.0.0.0 – 10.255.255.255. It is unlikely that someone is going to try to connect to a phone or laptop that’s connected over Wi-Fi, or rather, legitimately try to connect, it makes sense to use a private address range for that purpose, so, we’ll use 10.10.10.0 as our network.

A new host, let us say an Android phone, connects to the network. DHCP sends that host an address — 10.10.10.100 — a netmask — 255.255.255.0 — and a default gateway — 10.10.10.1.

We have 256 addresses from 0 to 255, and we take away three for the network, gateway and broadcast, leaving 253. Meanwhile, we get more people and more devices per person. Laptops, phones, tablets, and more come on, taking up all of those 253 addresses. So, we say we have room to grow, and add 10.10.11.0-255 to the 10.10.10.0-255 we already had, and the DHCP lease, the time where the DHCP server gives this IP address to this MAC address (the actual unique address of a network device), is up, so this Android phone gets a new address. 10.10.11.100.

00001010 00001010 0000101 0 00000001 - 10.10.10.1 - Gateway

00001010 00001010 0000101 1 01100100 - 10.10.11.100 - Host

11111111 11111111 1111111 1 00000000 - 255.255.255.0 - Netmask

When the netmask digit is one, the digits between two addresses should be the same, else they’re not in the same network.

And, if your gateway is not in the same network, how are you supposed to get out to the rest of the Internet?

So I reported this issue.

Weeks pass.

Paraphrase: “Go away kid, you bother me.”

And, a month later, the phone could connect. Maybe someone whose positon in the org chart was higher than mine complained. Maybe one of their devices lost a lease and was issued a new, out-of-network IP address. Maybe — gasp! — they read my email and fixed the issue. I will never know.

But that is a little on how IP works and what the netmask is for.

If you have any questions or comments, I would be glad to hear it. Ask me on Twitter or make an issue on my blog repo.