All of our discussion thus far has revolved around the idea of using IP address to contact other machines. However, isn't exactly an IPv4 or IPv6 address, but it certainly looks like I can somehow use that sequence of characters to contact another Internet-connected device. Remember, if you know the IP address of, you can open it up using a web browser: if you click here, you should be whisked away to CNN's homepage, despite the fact that your web browser's address bar has no references to Of course, remembering the words "cnn dot com" is much easier than remembering (and typing) "one five seven dot one six six dot two two six dot two six," so it'd be ideal to not have to waste brain cells on many sequences of 12 digits.

The Domain Name System, or DNS for short, allows us to give more convenient names to networked devices. DNS is essentially a huge phonebook distributed across many storage locations around the world. Just like the YellowPages that you wish weren't delivered to your front door, each domain name in the DNS database has an associated IP address. Whenever you try to access a domain name that isn't just an IP address, your web browser will use DNS to look up what IP address corresponds to the domain being accessed. Once we have an IP address, our request can proceed in the manner we've already seen! So, DNS is really just another networking layer that makes life a bit easier for us. However, unlike a printed phonebook, changing entries in DNS is pretty simple and relatively fast.

In the early days of the Internet, the big phonebook of IP addresses was simply a text file called HOSTS.TXT. Developers would routinely copy some authoritative copy of this text file to their own machines, so they could then take advantage of all domains on the Internet. Remnants of this system still remain even on modern operating systems. On my Mac (or Linux), I can open up a file called /etc/hosts that lists aliases for IP addresses local to my machine. On Windows, this file is located in C:\Windows\System32\Drivers\etc\hosts, but it's contents are the same. On each line, we have an IP address followed by a space and then a domain name. Let's try adding a new entry. is an IP address for, so add the following line to the bottom of your hosts file (you may be prompted for your administrator password, since this is a system file):

Now, open up a web browser and head to Toto, we're not in Redmond anymore. When we added that line to our local hosts file, we effectively created a new entry in the DNS phonebook that says " corresponds to the IP address" If you type that IP into your web browser, you'll see that it's simply one of Google's (many) IP addresses. So, when your web browser saw "," it was told that that domain resolved to one of Google's IPs, which led you to Google's homepage.

Given the massive size of the Internet, it's probably not the best idea to keep the list of all domain names in a single text file. Not only would that be impractical to keep everyone on the Internet up-to-date with the latest version, but if that file were ever compromised, someone could wreak havoc on the Internet. So, it makes more sense to distribute that information across multiple machines, called DNS servers. Rather than keep track of the entire domain name space, each DNS server typically maintains a smaller subset of all the domains that have been registered on the Internet and their corresponding IP addresses.

Let's say that I make a request to First, my browser will probably check its cache of recently-accessed domain names to see if it already knows the IP of Assuming it doesn't, it will contact a cache DNS server in order to ask what IP the domain "" points to. This DNS server is could be managed by my ISP, which is a company responsible for connecting me to the Internet. My computer was informed of the DNS servers on its network when it first connected (unless I manually set them myself), and on a Mac, I can view the IP addresses of these DNS servers in my network settings, as shown below. Because various computers on the network are making requests to the same DNS servers, there's a good chance that a given DNS server is going to be asked for the IP address of a popular site like "" pretty frequently. So, cache DNS servers will remember the IP addresses they've been asked for in order to give other computers making a request to the same place an immediate answer. If a given cache DNS server doesn't already know the IP for a domain, then it's going to have to keep looking by asking other DNS servers if they know the answer.

DNS Client Settings

If this DNS server knows where "" is, perhaps because another computer on the network made a request to a few minutes ago, it will respond immediately with the correct IP, and we'll be good to go. But because that makes the story boring, let's assume the first DNS server we contact doesn't know the IP address of Now, perhaps after asking a few nearby DNS servers and getting no result, this DNS server will go ahead and contact a root DNS server. Root DNS servers are located all around the world (click here for a complete list!), and they're responsible for forwarding requests along to a TLD DNS server. TLD DNS servers are responsible for handling requests to a particular TLD, so my request will be sent along to a DNS server responsible for handling requests for domains ending in ".net". While the TLD DNS server doesn't know exactly what the address of is, it does know the right person to ask. This server will proceed to query the authoritative name server for, which is responsible for maintaining a list of IP addresses for all addresses in a DNS zone, which is simply a group of domains. Now, this server can respond with the IP address for, which will finally make its way to my web browser. Phew! All of this before my computer even started to request data from!

Let's take a more concrete look at the process of resolving a domain name. If we head here, we can see the process of looking up the IP address of a domain name. Let's look up This particular tool doesn't happen to be using any DNS caching servers, so the first request goes straight to a root DNS server. After hitting one of the root DNS servers, such as, the request will be sent to a TLD DNS server responsible for .net domains. From this DNS server, like, the request will be forwarded to the authoritative DNS server, which for In this case, the IP address of this DNS server is, which corresponds to Finally, the authoritative name server returns the IP address for, which is Now, that information makes its way back down the hierarchy until we're finally informed of's IP address.

We can see, then, that DNS information is organized hierarchically. First, my computer checks its local cache of IP addresses, which could be maintained in a file like /etc/hosts or by the web browser itself. Next, we'll check a cache DNS server to see if we can immediately get an answer, but, just like your CPU cache, the data stored on cache servers could be ephemeral. Then, we have authoritative name servers that maintain IP addresses for groups of domain names, and at the top of the hierarchy are root DNS servers, which manage sets of domains.

Sometimes, the domain that we typed into our web browser's address bar hasn't been registered by anyone, so it doesn't actually point any IP address. If this is the case, then the gods of the Internet (aka the people who govern the Internet and write standards describing how devices should operate on the Internet) have decreed that your web browser should display some kind of message letting you know that this domain isn't owned by anyone yet. However, it turns out that many ISPs don't want to listen to the gods of the Internet. Instead of letting your web browser tell you that a site doesn't exist, an ISP may instead redirect you to a page that looks something like this:

Comcast DNS Hijack

Why might your ISP want to do this? $$$! As you can see from the above photo, this page contains advertisements from a number of different companies that paid Comcast to feature their clever adverts on this page. Issues like this are part of a larger debate referred to as "net neutrality." Essentially, the questions behind net neutrality ask what degree of control your ISP should exercise over your Internet connection. For example, rather than charge you one, flat rate to access the Internet, ISPs could instead create a tiered model where you pay for access to certain sites. As shown in the below (currently) fictional advertisement, you could pay $5/month to access search sites like Google and another $5/month to access news sites like CNN. While ISPs might advocate this kind of pricing scheme, others want the Internet to remain a more free environment where newer, smaller companies aren't put at any disadvantage to tech giants as a result of tiered pricing. Debates like these are currently ongoing, and right now, there isn't a straightforward answer!

Tiered pricing

Of course, even a distributed system like DNS can still be compromised, which would be pretty bad news for cats everywhere. So, a few years ago, ICANN, the organization responsible for assigning domain names, bestowed upon seven heroes the keys to the Internet. When all seven individuals, scattered across the globe in the US, UK, Burkina Faso, Trinidad and Tobago, Canada, China, and the Czech Republic, come together as one superpower, the Internet can be rebooted and rid of all evil. You might think I'm joking, but this actually isn't too far from the truth. DNSSEC is an emerging standard that seeks to make DNS more secure in general by ensuring that attackers can't forge the IP addresses of websites. Someone has to have the power to restart it!

While the first HOSTS.TXT file simply stored a series of mappings between IP addresses and domain names, modern DNS servers store a bit more information. Today, most domains have the equivalent of a spreadsheet associated with them. Here's some of the information that's currently associated with

DNS Manager

Each row in the table is called a DNS record. The table headings describe some of the different types of records that DNS uses. First, we have an SOA record, which specifies the authoritative information about the zone, including the primary name server. Next, we have NS records that specify additional name servers for the zone. MX records (Mail eXchange) specify how email sent to the domain should be handled—more on this one in particular a bit later! Next up we have A records, which define the IP addresses of the domain. This is essentially what we created when we edited our hosts file a little while ago. Here, we can see that both and point to the same IP address, which effectively makes typing the "www" part of the website's address optional. As an aside, the AAAA records referenced above are used for IPv6, while A records are used for IPv4 (remember the difference?). Finally, we have CNAME records, which allow us to alias domains to other domains. In this example, has been aliased to Each of these rows also has a TTL, or time-to-live, which defines the amount of time that should pass before the record should be refreshed by a DNS server. While my hosting service provides a nice web interface for editing DNS information, all of this information actually exists in a text file called a zone file located somewhere on my hosting company's infrastructure.

While surfing the web, you may have noticed that most domains are in the form domain.tld, where "tld" is a small set of suffixes including com, net, and org. The TLD part of a URL is known as the top-level domain, which comes from a list of about 250 suffixes approved by the Internet Assigned Numbers Authority (IANA). Addresses like foo.domain.tld called subdomains, and allow a domain to be divided into separate components. In terms of DNS, I can assign different IP addresses to different subdomains for the same domain name by creating A records with different values. Based on the last row of our example above, I can also create a subdomain using a CNAME record. Here's a list of some common TLDs and their intended usages. However, today people often purchase TLDs based on what's available or aesthetically pleasing, as some TLDs have no restrictions on who can purchase them and how they can be used., for example, was already taken by someone else, so I instead purchased and However, I wouldn't consider CSCI E-1 a "network," which was the intended usage of the ".net" TLD! Some TLDs, on the other hand, cannot simply be purchased by anyone on the Internet.

TLD Intended Usage Open to anyone?
.com Companies Yes
.edu Educational institutions No
.gov US government entities No
.info Information Yes
.mil US military organizations No
.net Networks Yes
.org Organizations Yes

The IANA also defines a list of TLDs based on country codes, called ccTLDs. While this is meant to give countries their own little piece of the domain namespace, many people take advantage of country codes to create cute domain names. For example, the ccTLD for Libya is ".ly," which is used by URL-shortening services like and There's also ".me," the ccTLD assigned to Montenegro, which is cleverly used by sites like, which allows you to create your own home page. Even the United States' own ccTLD, ".us," has been put to good use by sites like I'm also proud to say that my initials are a valid TLD, thanks to Turkmenistan. However, since ccTLDs are managed by individual countries, real-world politics can sometimes oddly come into play. Recently, a tech startup (then) called had to change domains because of a conflict in Syria! You don't see that one every day.

Country code TLDs are nice, but they're still a bit limiting in terms of the domains companies and URLs can create. To solve this problem, the IANA is considering introducing a new set of between 100–3000 TLDs based on an application process. For a cool $185,000, you can apply to create your very own TLD, though the IANA estimates that these new TLDs won't be created until the end of 2013 due to possible trademarks concerns. However, if the application for a TLD is granted, then the organization that applied will have full control over who can and can't use the TLD. ICANN's website has an up-to-date list if you're curious what's already been applied for!

Purchasing a domain is actually pretty easy (and inexpensive!). To do so, all I need to do is head to a domain name registrar like GoDaddy, Namecheap, or Network Solutions. These companies handle interfacing with ICANN, the non-profit organization responsible for managing the huge number of registered domain names, among other things. Domains typically cost between $10–$15 per year to maintain, which I think is a pretty reasonable price for carving out your own little place in the Internet. While some registrars may include some number of email addresses on the domain your purchased, simply buying a domain name is separate from creating a website. After all, we now know that a domain name is simply an alias for some IP address, which by nature has to be attached to some Internet-connected and publically-accessible device! Unfortunately then, hosting a website usually occurs a monthly cost in addition to the annual fee associated with a domain name, but such is life on the Interwebs. More on hosting your own website later, though!

Often though, what we type into our web browser's address bar is much more than simply a domain name. Usually, we locate pages on the Internet using URLs, or uniform resource locators. In general, URLs are in the form scheme://domain:port/path?query_string#fragment_id, and the goal of a URL is to identify a specific resource on the Internet. is one website, but it contains many different resources, including the problem sets page and the announcements page. Each problem set document, which has a unique URL, can also be considered a resource, and so can the various images that appear on the site, since they can be accessed via a URL as well. The word "resource" sounds a bit abstract at first, but you can think of a resource as something that you want to access on the Internet, whether that be a video of a cat or a web page of cat facts.

A URL can contain several different pieces, so let's look at an example: The scheme describes how information will be transferred between my computer and the machine responding to me, and the username and password can be used for some sites containing authentication. Here, we're using a scheme called HTTP, which we'll look at in much more detail in the next section. Next is the host, which is the domain we're contacting, followed by the port we're connecting to (much more on ports soon!). The path, in which hierarchical components are separated by slashes, comes next. As you can see, the path in a URL looks just like the paths to files on your located on your computer; in some cases, a URL path indeed represents at least part of the path to a file located on a remote machine. Finally, additional information can be sent along using the query and fragment. Typically, the query consists of key-value pairs that are passed with the request, while the fragment usually contains additional information used by the web browser. A key-value pair simply associates values, which could be numbers or words, with keys, which are simply identifiers for those values. For example, a key-value pair like "foo=bar" says that I want to send along the value "bar," and whoever is handling my request can access that value "bar" by asking the question, "what is the value for the key 'foo'?"

You may also have heard the term URI used to describe what you type into your web browser's address bar. URIs and URLs are actually different things, though the two terms are commonly conflated. URIs, or uniform resource identifiers, are actually more general than URLs, as they serve simply to identify something, not necessarily locate something. The "L" in URL does indeed stand for "locator," and that's because the purpose of a URL is to describe how to find some resource on a network. In doing so, a URL can also serve as a URI for that resource, since we can say that a resource's location identifies it. For example, saying that my name is "Tommy MacWilliam" identifies me, but it doesn't give any information about how to locate me. On the other hand, the address "33 Oxford St., Cambridge, MA, USA, Planet Earth" both describes the location of a building and identifies it (since there is one building with that address). So, a URL is also a URI, but a URI isn't necessarily a URL. If you head to your local library or bookstore, you can also find an example of something that is a URI but not a URL: ISBN numbers. If unfamiliar (because who reads books anymore anyway?), an ISBN is a unique number assigned to published books; an ISBN for one of my favorite books, Alice's Adventures in Wonderland, is 9780811822749. Formally, this is called a URN, or uniform resource name, and is officially expressed as urn:isbn:9780811822749. Again, this ISBN number certainly identifies this great book, but it doesn't tell you where you can go buy it.

Sounds like URIs are kinda abstract, so let's come back to URLs, which we type into the address bar every day. We said before that an image on a web page or a problem set PDF is a resource, but many URLs don't actually map to a file on disk. A URL that is a part of a web API, or application programming interface, could be one example. API is a pretty common buzzword in the tech word today, and an API (in the context of web applications, anyway) is simply a way of exposing information via a standardized structure of URLs. By creating an API, or a set of URLs other people can access, a site can make information available in a more dynamic way than creating actual files on disk. Facebook, for example, has an API that allows you to get information from users' Facebook profiles, and Google has APIs that allow you to access your events on Google Calendar or search YouTube.

Let's take a look at an example of an API. The MBTA (the organization that runs the Boston subway system, if you're unfamiliar), has a cool API that makes real-time train information available. For example, if we head to this URL, we can see a live schedule for the red line. However, this page doesn't look as shiny as a Google Map showing train locations. Instead, this data is formatted in JSON, which is a standardized, machine-readable encoding of information. Now, other software developers can use this information to build cool apps! While this URL might point to an actual file called "red.json" somewhere on an MBTA server, since the positions of trains are constantly changing, there's a good chance that this URL is more dynamic than simply a static file. A bit later, we'll see how this might actually be implemented!

Alright, let's take a step back and tie together what we've seen in the past couple sections. When we make a request to a web page like CNN, we probably do so using its domain name, So, the first thing your web browser needs to do is figure out the IP address that corresponds to This is where DNS comes in. Your web browser will make a request to the IP address of a DNS server, which may or may not know the IP of If it doesn't, then it will ask a root DNS server, which will then query an authoritative name server to get the IP address for This response will eventually make its way back to your web browser. Now that your web browser has an IP address, it's ready to request some data. However, your computer isn't directly connected to the computer behind that IP address. Instead, the request will be forwarded along to a router on the same network as your computer. This router probably doesn't know where is, but it has an entry in its routing table that tells it where the next nearest router is. So, the router will forward the request along to a router that is closer to the final destination, and eventually, your request will reach its final destination!

We still haven't seen, though, how exactly your request is sent, and how sites like know what you're looking for. Before we discuss that, let's take a brief detour through Internet history!

  1. Why is it important that root DNS servers are kept secure? What could happen if one were ever compromised?

  2. What's the difference between an A record and a CNAME? When might you want to use one over the other?

  3. When you register a new DNS record, there's a good chance you won't be able to access your new domain or subdomain immediately. Why not?

  4. I'm lucky enough to say that my initials are a recognized TLD! (I'll have to fly to Turkmenistan to thank them someday.) Can you say the same for your initials?

  5. Let's talk about net neutrality. If you're unfamiliar with the topic, you might want to curl up with some readings like,, and In a short paragraph, tell us where you stand on the issue and why! To get you thinking, consider how net neutrality affects economics, freedom, and fairness from an Internet user's perspective!

  6. Why might a company like Google or Facebook want to expose their data via an API?

  7. You just typed into your web browser's address bar and hit Enter. In a technically detailed paragraph, what happened? You can assume that you are indeed connected to the Internet and that URL points to a valid page on YouTube (which it does, so you should click it). Your response should reference terms like routing and DNS!