The last couple of posts in this series have dealt with how Ethernet frames and IP packets get to their destination. In this post, we’ll step up a little bit and look at the role DNS plays in getting those packets to the correct IP address. We’ll see how this works in general and the issues specific to a Hyper-V environment.
Its usage has expanded over the years, but the basic problem that DNS addresses is that humans are not very good at remembering numbers. We have a comparably good ability to recall words, especially when using associative techniques. Computers, as you’ve probably noticed, have the opposite problem. Not only do they like numbers, they store “words” as numbers. DNS was designed to bridge this gap.
At its core, DNS (domain name system) is a directory that matches names with numbers. That’s about as complicated as it gets. It has a few other functions, but they all boil down to matching a name to a number. There are a couple of common analogies that easily illustrate this concept.
The first is phone numbers. Each phone line has a globally unique number. Remembering more than a few of them is extremely difficult. So, phone directories were created. For most of my life, these directories were manifested in large printed books known as the “white pages”, which first lost ground to basic speed dial, and are now built right into everyone’s smart phone.
The second are physical street addresses. In the U.S., the ZIP+4 alone specifies the exact destination. It’s usually accompanied by a larger, more complicated arrangement that is more human-comprehensible.
Just as those systems match human-decipherable addresses to more specific locations, DNS matches human-readable names to IP addresses. Like the telephone system, it does so in a precisely ordered, hierarchical fashion. This system works from right-to-left. As you work toward the left, each successive element becomes more specific. In a fully-qualified domain name, the left-most object represents the final destination. We’ll work with the TechSupport website as an example:
Each element is separated by a dot. In a complete DNS name, a dot does exist at the very right-most position, although it’s usually suppressed. This dot represents the root domain; nothing can be any higher in the hierarchy. This root domain contains all other domains.
Moving to the left, the next object we encounter is “com”. This is known as a top-level domain, which is occasionally abbreviated to TLD. “com” is one of several well-known top-level domains that are available on the Internet. Recently, changes to the public registration system have added quite a number of additional top-level domains. Some of these are protected, like edu and gov. Others are open to registrars to provide naming services to customers.
After “com”, we find another period. The periods are how DNS separates elements, but have no special meaning in any position other than the root domain.
Next, we reach “techsupportpk”. The contents of this element are generally just called the domain. In this case, the company called “Techsupportpk” has registered the domain name of “techsupportpk” underneath the top-level domain of “com”. As the legal registrants, they can put just about anything inside this domain that they like and no one else is allowed to use it.
The next element is “www”. I believe that this particular character string is largely responsible for most of the confusion that people feel when talking about DNS. Because it is the left-most element, it is the most specific part of the entire DNS name. In a traditional fully-qualified DNS name, this would refer to a single computer called “www”. However, since the dawn of the web page, it seems that everyone has a computer called “www” sitting on the Internet, serving up web traffic. It is the ubiquity of computers named “www” that confuse people when you tell them that it is the most specific element of the web name, not the least.
In traditional DNS parlance, the “www” portion is usually called the hostname. Of course, in the modern era, there probably isn’t a computer actually called “www”. It’s probably a hardware load-balancer or a group of computers operating in round-robin or something of the sort. But the point is, “www” is a singular entity that has a specific IP address that is the target of the fully-qualified name “www.techsupportpk.com”.
So, to match it back to the street address analogy, the right-most period means “the world”, “com” means the country, “techsupportpk” is the city, and “www” is the street and house number. To match it to the phone number analogy, the right-most period is again “the world”, “com” is the country code, “techsupportpk” is the area code and prefix, and “www” is the final set of numbers. These analogies are obviously not completely perfect, but the concepts are the same.
DNS becomes considerably easier to understand when you move away from “www”. Another DNS name that’s common to many Internet entities is “support”. For example, “support.microsoft.com” refers to an entity named “support” that is part of the domain “microsoft” that is a member of the TLD “com”.
A complete URL also contains a resource identifier. For example, http://www.techsupportpk.com/support.php refers to a resource named “support.php” that is served by the entity named “www” on the domain “techsupportpk” which is a member of the TLD named “com”. Most web sites employ default resources, which means that they automatically serve up a specifically-named entity, such as index.html whenever the client browser doesn’t request a resource. In terms of a web server, such as resource is always delivered in some fashion. For other URLs, such as ldap://dc1.domain.local, just access to the target system may be enough. The important fact for this discussion is that a trailing slash (/) and anything that comes after it is not part of the DNS name.
The above refers to the entity named “chat1” which is a member of subdomain “chatnet” which is a part of the domain “socialmediaco” which is under the TLD “com”.
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- Part 1 - Mapping the OSI Model
- Part 2 - VLANs
- Part 3 - IP Routing
- Part 4 - Link Aggregation and Teaming
- Part 5 – DNS
- Part 6 - Ports, Sockets, and Applications
- Part 7 - Bindings
- Part 8 - Load-Balancing Algorithms
DNS
DNS is a remarkably simple, yet just as remarkably misunderstood technology. I’ve lost track of the number of truly brilliant people I’ve met that struggle with it. So, if you’re confused by DNS, you’re in good company. Let’s see what we can do to get rid of the confusion.Its usage has expanded over the years, but the basic problem that DNS addresses is that humans are not very good at remembering numbers. We have a comparably good ability to recall words, especially when using associative techniques. Computers, as you’ve probably noticed, have the opposite problem. Not only do they like numbers, they store “words” as numbers. DNS was designed to bridge this gap.
At its core, DNS (domain name system) is a directory that matches names with numbers. That’s about as complicated as it gets. It has a few other functions, but they all boil down to matching a name to a number. There are a couple of common analogies that easily illustrate this concept.
The first is phone numbers. Each phone line has a globally unique number. Remembering more than a few of them is extremely difficult. So, phone directories were created. For most of my life, these directories were manifested in large printed books known as the “white pages”, which first lost ground to basic speed dial, and are now built right into everyone’s smart phone.
The second are physical street addresses. In the U.S., the ZIP+4 alone specifies the exact destination. It’s usually accompanied by a larger, more complicated arrangement that is more human-comprehensible.
Just as those systems match human-decipherable addresses to more specific locations, DNS matches human-readable names to IP addresses. Like the telephone system, it does so in a precisely ordered, hierarchical fashion. This system works from right-to-left. As you work toward the left, each successive element becomes more specific. In a fully-qualified domain name, the left-most object represents the final destination. We’ll work with the TechSupport website as an example:
www.techsupportpk.com.
www.techsupportpk.com.
www.techsupportpk.com.
www.techsupportpk.com.
www.techsupportpk.com.
www.techsupportpk.com.
So, to match it back to the street address analogy, the right-most period means “the world”, “com” means the country, “techsupportpk” is the city, and “www” is the street and house number. To match it to the phone number analogy, the right-most period is again “the world”, “com” is the country code, “techsupportpk” is the area code and prefix, and “www” is the final set of numbers. These analogies are obviously not completely perfect, but the concepts are the same.
DNS becomes considerably easier to understand when you move away from “www”. Another DNS name that’s common to many Internet entities is “support”. For example, “support.microsoft.com” refers to an entity named “support” that is part of the domain “microsoft” that is a member of the TLD “com”.
Confusing Conventions
The usage of “www” isn’t the only way that people trip up over DNS. Another is that we’ve also developed tools, especially web servers and browsers, to reduce the complexity in a way that masks the true operation of DNS. For instance, if you just tell your browser to go to “techsupportpk.com”, you’ll land on “http://www.techsupportpk.com”. Some websites, such as http://sourceforge.net, mask the computer name out entirely. This very common behavior has led people to believe that they are connecting to “techsupportpk.com” and that the “www” is just an optional relic. Even though the actual name of the system you’re connecting to really doesn’t matter, it’s still there and is separate from its containing domain name.What about https:// and other URL components?
Another stumbling block for many people is the fact that a DNS name is just one part of a uniform resource locator (URL). A fully-qualified DNS name is composed of nothing more than alphanumeric identifiers (hyphens are also acceptable) separated by periods. The other elements of a URL have different purposes and are not part of DNS. The first of these elements that you usually encounter is the protocol identifier, such as “http://”. Other common protocol identifiers are “https://” and “file://”. What these do is identify to the browser (or any URL-friendly application) that it should connect using a specific protocol. As with “www”, “http://” is so ubiquitous that it’s sometimes assumed to be attached to the DNS name somehow.A complete URL also contains a resource identifier. For example, http://www.techsupportpk.com/support.php refers to a resource named “support.php” that is served by the entity named “www” on the domain “techsupportpk” which is a member of the TLD named “com”. Most web sites employ default resources, which means that they automatically serve up a specifically-named entity, such as index.html whenever the client browser doesn’t request a resource. In terms of a web server, such as resource is always delivered in some fashion. For other URLs, such as ldap://dc1.domain.local, just access to the target system may be enough. The important fact for this discussion is that a trailing slash (/) and anything that comes after it is not part of the DNS name.
Subdomains
The above examples are the most common and basic format for a DNS name. The dotted notation can continue to proceed to the left with subdomains. Consider:chat1.chatnet.socialmediaco.com
Fully-Qualified and not-so-Fully-Qualified
In the early days of DNS, you really only thought about hostnames, domain names, and fully-qualified domain names (FQDN). For an individual host, the first two would combine to form the latter. So, going back to our original example, “www” is the hostname, “techsupportpk.com” is the domain name, and “www.techsupportpk.com” is the FQDN for the “www” host. But, as work was done to simplify even the DNS system for the average user, other things began to be lost. To reuse another example, http://sourceforge.net is missing a hostname. http://intranet is missing a domain name. Any time you see a DNS name that appears to stray from the format shown above, just be aware that it’s likely that you’re looking at a name that is somewhat less than fully-qualified. There are methods employed that take these partial inputs and resolve them into FQDNs. We’ll look at these later.DNS Server Operation
If you have access to a Microsoft DNS system, I find that DNS is pretty simple to understand just by poking around in the console. Here’s a screenshot of mine: