How Servers Serve Content

Computers are a great tool and provide many perks to their users; they can perform tasks in a rather impressively fast manner, potentially offer paperless offices, and you’re even able to play computer games…of course not during working time! Aside from these great benefits, however, how useful would they be if we typed something up, had to print it (thus negating the “paperless” aforementioned attribute) and fax / mail / hand-deliver it to the respective recipient?

This is where the ever-loved “network” aspect comes in. There is history to how this concept began, was revised, and came to be what it is today; read this if you’re interested in its history. Regardless of its ancestry, I am confident that most anyone is thankful to the many contributors to this awesome ability. How else could we send emails virtually versus the above-mentioned manual methods? What about the massive online gaming platforms that allow virtual users to face each other in combat, or team up and conquer dungeons and whatnot? Or in the case of our team’s admiration for “Neopets,” how would we be able to access their awesome website to do whatever we wanted to do?

The goal of this post is to clarify what a network is, some different types of networks used in today’s world, different devices used in networking, as well as how data is carried between the devices in terms of network media. Perhaps in the future I will build on this and dive into the Open Systems Interconnection (OSI) model where I will break down each of the seven layers. Stay tuned for that, but for the time being let’s dive into the basics of networking.

What Is Networking?

To be clear, networking refers to the ability for multiple network nodes to communicate with each other and share resources. Network nodes refer to any network-connected electronic device; network-capable gaming devices, cellular devices, laptops, computers, etc. Any sort of interaction from one network node reaching out to another network node is considered a network, regardless if there is Internet access available or not.

Two common implementations include peer-to-peer (P2P) and client/server set-ups. As the names may imply, a P2P connection is one where network nodes are able to connect to each other and share resources. Workloads are shared between nodes and no one has more privileges than the other. Think of this as a way for any number of nodes to share files with one another, but not designated to do so. In contrast to this is client / service architecture, which dictates one server to be the storehouse of whatever content. With this set-up, you have one computer that may have more resources and is a bit faster at processing requests.

The key difference between the two architectures is that in P2P set-ups, all nodes are equal in privileges in accordance what the other nodes allow access to. Client / server architectures allow for more granular, centralized control. The two can seem ambiguous depending on your perception, but there is definitely a difference between them.

Types of Networks

On a smaller scale, local area networks (LAN) entail networked electronic devices that are within the same vicinity, generally. An example of this would be a home or smaller office. You can obviously get way broader than this, an example would be a wide area network (WAN), which is essentially what the Internet is. Regardless what set-up you have, chances are you have an Internet service provider (ISP) running some sort of cable (copper wired Ethernet or glass-based fiber optics) from their datacenter to your home or office. From there, you’d connect a modem to modulate outgoing network signals, and demodulate incoming network signals. The modem then connects to your router that may connect to a switch, which allows multiple devices to connect – thus creating a LAN. I’ll cover network devices a tad more in another section.

For corporations or businesses with multiple offices in remote areas, the HQ office can connect to these offices via a virtual private network (VPN). A VPN is essentially an encrypted tunnel that is created over the unencrypted Internet pipeline. Bottom line is, if you want to be secure between HQ and remote sites or home-based employees, use a VPN. Another important set-up to touch on is virtual LAN, or VLAN. In short, VLAN refers to creating segmented networks, thus allowing a segregation between user sets. Essentially you can have a VLAN domain for servers, one for the HR department, another for your IT team, and a general user VLAN. There are many other types of networks, but these are the main varieties.

Networking Devices

Starting from the gateway, that is the device that allows access to and from the Internet. There are devices that combine both modem and router capabilities into a single device. As stated previously, a modem’s job is to modulate outgoing network traffic, and demodulate incoming traffic – hence modem. On the other hand, a router is what routes traffic from one network node to another – whether on the local network or somewhere out on the Internet. It is important to note that routers specifically route between different network segmentations. As an example, if node A has an IP address of 192.168.1.5 and wants to connect to node B’s IP address of 10.0.1.5, then a router would assist in this.

Some routers possess the ability to broadcast wireless frequencies, thus allowing mobile devices, laptops, and other electronic devices to connect via wireless means. Some devices don’t even have network interface cards (NICs), so wireless connectivity is crucial in order to get online. If your router doesn’t have this ability, then a separate access point (AP) would allow for this functionality. In this case then the AP itself would be hardwired to the router. Wireless frequencies have gotten better and stronger over the years, but they still do face obstruction the further away your wireless device is compared to the AP; walls, plumbing, or whatever may be between the line of sight are other examples of this.

Let’s back up a bit to routers and connecting devices to them, such as the AP above. Routers have few Ethernet ports, thus limiting the number of hardwired devices. If you don’t have many devices, no worries, but if you do, then you may want to consider a switch. Modern technologies have paved the way for switches, which are generally faster than the predecessors they’ve replaced – hubs. Seeing that hubs are pretty much a thing of the past, I’ll explain switches but leave it up to you to research hubs more if you’re into the history of older devices. Switches essentially extend the router’s NICs to more than what is integrated on the router itself. Further, they also remember networked devices based on media access control (MAC) addresses to negate the need to send data frames to all devices, are able to send and receive transmissions simultaneously (known as full-duplex), and have the ability to send more data per second than hubs.

Networking Media

I touched on this throughout the article a bit but wanted to have a section for this as well. Network nodes communicate in terms of network media. Physical media refers to the more common Ethernet cables, optical fiber, and coaxial cables. Wireless technologies include infrared and radio waves, which introduces Bluetooth options, as well as satellite connections and the wireless capabilities of cellular devices.

Conclusion

This article covers only the basics of what networking is, different network implementations, as well as network devices and media. If you’re a more technical person, what this post covers can translate into a few layers of the OSI model – mainly layers one (Bluetooth & NICs) and two (switches & APs), but a tad into three when covering routing between networks (routers). By no means is this a super technical break down of the layers, though potentially one day in a future post I will go there.

by Rik Farrow and Scott Pinzon, with updates by Corey Nachreiner

[Editor's note: This article continues our attempt to explain the complex subject of subnetting in terms you can understand even if you're not a rocket scientist. If these concepts are new to you, you'll increase your comprehension by reading Corey Nachreiner's prequel, "Understanding IP Addresses and Binary," and then Rik Farrow's "Understanding IPv4 Subnetting (Part 1)."]

In Part 1, I explained how an IP address is really two addresses in one: part of it is a network address, and part of it is a host address. This split between the network and host portions of the address formerly occurred predictably, on full-byte boundaries. But newer addressing techniques, aimed at stretching the finite number of addresses available to the ever-growing Internet, now allow an IP address to be split anywhere within its 32 bits.

Because of the history of TCP/IP, your operating system (which provides you with TCP/IP) still expects you to use the old Class system. If you use an address in the Class C range, say 192.168.1.0, your operating system will automatically think of it as a /24 address: the first three bytes indicate a network address. But suppose you don't want 192.168.1.0 to be a /24 network. Suppose you want it to be a /26. How do you, the administrator, impose your iron will upon IP addressing? By specifying a subnet mask. Your network has always had one. But in this case, you are seizing control by specifying one that differs from the Class C default.

To understand how a subnet mask works, we have to go back to binary, because that's what your computers understand. So, in binary, 192.168.1.0 is:

11000000.10101000.00000001.00000000

(If you don't understand where we got that value, see Corey's article.)

The subnet mask is simply a binary value that uses ones to represent how many bits (out of the 32-bit IP address) represent the network portion of the address. Zeros indicate the host portion of the address. So if you want a /26 address, you want 26 bits of the IP address to indicate the network and 6 bits for indicating host addresses, written like this:

11111111.11111111.11111111.11000000

To change a Class C address (or any class of address) into a classless CIDR address, you supply a subnet mask which differs from the "classful" default subnet mask. Your computer uses this subnet mask to "mask off" the network portion of the address, reminiscent of the way an usher in a theater might put velvet ropes around rows of seats reserved for a large group. The easy way to understand masking is to count the number of one bits in the subnet mask, because that is how many bits of the IP address will become the network portion. The hard way is to imagine how your operating system does this: by using a CPU operation known as a Logical or Binary AND. Without plunging deep into math theory -- in other words, defer knowing "why" -- let's stipulate that in this operation, 1 + 1 = 1, and 1 + [anything else] = 0. If that's hard to wrap your head around, think of 1 as "true" and zero as "false." The truth plus more truth equals truth. The truth plus anything false is no longer the truth. Using this Boolean logic, combining our sample IP address (192.168.1.0) with our sample subnet mask (/26) produces this result:

11000000.10101000.00000001.00000000 + 11111111.11111111.11111111.11000000 11000000.10101000.00000001.00000000

Convert the answer back to decimal, and you still have 192.168.1.0 -- but because you've specified a non-standard subnet mask, your OS now understands that this is 192.168.1.0/26, and that this network's range is 64 addresses rather than 256.

Bits of Pain

You don't have to go through the painful labor of calculating binary every time you want to specify a subnet. Though your OS uses binary bits internally, your OS permits you to specify the subnet mask in decimal or in slash notation. If you'll recall Corey's article, a full byte with all eight bits turned on (11111111) adds up to a value of 255 (128 + 64 + 32 + 16 + 8 + 4 + 2 + 1), so when subnet masks are expressed in decimal, you'll often see some 255s. For example, the decimal representation for /16 becomes 255.255.0.0.

So, just to create another example, what is the subnet mask for a /22 address? Well, first there will be 22 ones, and the rest zeroes, or 11111111.11111111.11111100.00000000, or 255.255.252.0. Converting the binary into decimal can get tricky, so it's nice to have a calculator or computer program that does the math for you, or a table. You can find such "cheats" all over the Web, including these:

• Online Subnet Calculator
• Free Subnet Calculator for Windows
• Free Subnet Calculator for Mac
• Subnetting Cheat Sheet

This is also why slash notation is so nice (no binary-to-decimal conversions).

Why Subnet?

If you've followed the logic thus far, you might wonder, "Was this trip really necessary?" The answer: yes, because subnetting lets you split a single network address into many subnets.

Remember, every time an individual host on your network receives another packet of data, the one thing that host wants to know is, What do I do with this? For example, take a typical networked host, called Gandalf. Gandalf assesses every outgoing packet's Destination field in its IP header to decide whether the packet's destination is "local" or "not local." If the destination is local, Gandalf consults an ARP table for the MAC address of the destination machine, then sends the packet to it. If the destination is not local, Gandalf forwards the packet to his all-knowing buddy that handles everything else, a device we refer to as the default gateway. (This is some kind of device that routes, whether an actual router or something that can perform routing, such as your Firebox.) How does Gandalf define "local"? Simple: local is any device having the same subnet address as Gandalf.

Subnets are used throughout the Internet. But you can decide to use subnetting yourself on your own network, if you want. You can subnet for any reason, including security reasons -- for example, to split a single network into two or more parts, using one or more routers to connect each subnet together. Using routers means that broadcasts will (generally) be limited to each subnet. You could also use the Optional and Trusted interfaces of a Firebox to isolate two subnets, even if they were both internal networks. You can subnet to ease troubleshooting. Tracing a problem on a network segment is easier if the network has been cut into halves through subnetting; you're dealing with fewer hosts at a time.

Though at one time subnetting implied breaking your network down into shorter physical segments, today subnetting is not necessarily tied to the physical world. You can subnet simply because you want each department (or remote office, or floor in your highrise, or building on your campus, or ...) to have its own subnet address. You can decide, for example, that anything beginning with 192.168.100 indicates mobile users, and, say, 192.168.200 indicates executive users. You can make wireless addresses instantly recognizable to you, so you can keep an eye on them in your logs easily. Or whatever -- use your imagination!

But don't start yet...

Before you begin slicing and dicing your own subnets, we have to take another side adventure into the history of TCP/IP. Note that even though there are 256 possible addresses when we use one byte (0 through 255), you will only be able to use 254. This is because an address of all ones, or 255 in this case, is reserved for the broadcast address and an address of all zeroes is reserved for the network address. The broadcast address means "Send to all hosts on this network segment," so it obviously cannot be used as an individual host address. The network address is reserved for more traditional reasons.

As an example, suppose someone wanted to split their Class C network up into four smaller CIDR networks. Dividing the 256 possible addresses in a Class C network by 4 tells us each subnet can have 64 addresses. (However, for the reasons I just cited, only 62 of those addresses can be used for hosts.) Instead of using a Class C subnet mask of 255.255.255.0, or /24, you would use 255.255.255.192, or /26. This works because it leaves six bits (out of the 32 bits in an IP address) for addressing hosts. Six bits (32 + 16 + 8 + 4 + 2 + 1) is enough to make 64 addresses (the bits total 63, but 0 also counts as one of the addresses).

In Part 1, I pointed out that an IP address is two addresses in one: a network address, and a host address. When you specify a subnet mask, you could say that the portion that defines the subnet has been "borrowed" out of the host address portion and added to the network portion. Thus, the router that connects your subnets can determine which addresses belong to which subnet by considering the first two bits of (what was) the host address as being part of the (newly changed) network address.

How does IP addressing look when you slice up a network this way? Let's use 192.168.1.0/26 as the Class C network address to split up.

The routing for this will work whether all four subnets are on the same physical wire or not, as long as you have at least one strategically placed router that knows what's going on.

The binary arithmetic involved in using CIDR block addressing is painful. It is much easier to start with another private network address space, say 172.16.0.0, and use the third byte to represent internal subnets: 172.16.1.0, 172.16.2.0, 172.16.3.0, and so on. Now you use the default ( or "classful") subnet mask of 255.255.255.0 (or /24) and each subnet includes 254 host addresses. See how much easier it is to start with a Class B (/16) network, and split it into 254 Class Cs (/24)? Perhaps the designers of TCP/IP weren't so dumb after all when they chose to use byte boundaries in the initial network addressing scheme.

Subnets aren't black magic — just a place where administrators come face to face with binary arithmetic, often for the first time. We've barely scratched the surface of this topic. But at least now when networking experts are having one of those subnetting conversations where the technical concepts seem to rush past you at superhighway speeds, you have an onramp. You've got enough basics to join in the fun. Use your new powers wisely!

by Rik Farrow, updates by Corey Nachreiner

[Editor's note: If the difference between a bit and a byte stumps you, or if the term "IP addressing" seems foggy to you, you'll understand and enjoy this article much more if you first read "Understanding IP Addresses and Binary."]

Sure, you've heard of subnets. But if a crazed IT veteran glared at you and demanded, "Explain subnetting right now, or I'll have you fired for incompetence!," could you tell him much more than the vague idea that subnets have to do with TCP/IP networks? This Security Fundamentals article explains what IPv4 subnets are, what a subnet mask is, and how you might use them within your own network. In brief, subnets have to do with routing, but it'll take some explaining getting to that point -- enough that this will be a two-part article. (So try to avoid crazed IT veterans until you've read both parts.) By the way, subnets apply (in much the same way) to the more modern IPv6 protocol as well, but these articles will focus on IPv4 since that’s what most administrators still use.

Long ago, in a project far away...

Once upon a time, the entire Internet was very small. It was so small, in fact, that the entire Internet (then called the ARPANET) could use one byte, with a maximum value of 255, for addressing every connected computer. During this time, a group of graduate students working on a project at Stanford began writing a replacement for NCP, the early ARPANET protocol. They called their protocol TCP/IP, and allowed what they considered a generous size for addresses, four bytes.

What's more, they formatted the addresses so that the beginning of each address indicated which of three "classes" an address belonged to. The students had the notion that the Internet would be composed of a few very large networks (which they dubbed Class A), a larger number of medium sized networks (Class B), and many small networks (Class C). In the students' scheme, there were 126 Class A network addresses, 64,000 Class B network addresses, and almost 2 million Class C network addresses.

This construct didn't work out too well, because a Class C network could only contain 254 individual device addresses—a number restrictive enough that any medium-sized organization wanted a Class B all for its very own. By the early '90s, just as the Internet was becoming commercialized, the American Registry for Internet Numbers (ARIN) was running out of Class B network addresses.

Addressing the issues

(when addressing is the issue)

In the students' scheme, some of the four bytes of an IP address identified a network. The remainder of the address identified a particular machine, or host, on that network. In other words, any IP address is composed of one part network address and one part host address. In that sense, you could compare IP addresses to phone numbers. For example, if you wanted to call the US White House from Japan, you would dial 001 1 202 456 1414. The 001 1 represents the code to call from Japan to the US, 202 the area code, and 456 1414 the specific phone number. 001 1 202 corresponds to a network address, and 456 1414 to a host address.

But how much of a 32-bit IP address is the network address, and how much is the host address? The answer is a hearty "it depends." The student's addressing scheme split IP addresses into network and host addresses on byte boundaries. For example, a Class A address used the first byte to identify the network, and the remaining three bytes to identify individual hosts. Thus, in the address 126.10.11.12, the first byte (126.) is the network, and 10.11.12 is a host on the 126 network. Class B IP addresses, like 128.16.7.4, use the first two bytes (128.16) as the network address, and the last two bytes (.7.4) as the host address. In a Class C address, like 204.176.22.1, the network portion of the address takes up three bytes (204.176.22), leaving only a single byte for the host portion (.1).

Note that using a Class A address, you could address a mammoth number of hosts. Remember from the "Understanding IP Addresses and Binary" article that one byte can represent any value from 0 to 255? That means if you have three bytes available for identifying hosts, you could assign hosts IP addresses of 0.0.1, 0.0.2, 0.0.3, and so on, up to 255 (at least in theory; in practice, 255 is reserved for a special purpose). Then you could start over using 0.1.1, 0.1.2, et cetera. Working through all the possible combinations, the total amount of unique host IDs available in a Class A address is roughly 255 x 255 x 255 -- well over 16,000,000 available addresses! Following the same logic, a Class B network has two bytes left for host addresses, and two bytes allow us to address more than 65,000 hosts (255 x 255). And now you probably can grasp why a Class C address, using only one byte for addressing individual hosts, can only handle 254 devices. Why not 255 devices? I'll explain that in Part 2 of this article.

Given a generic IP address nnn.nnn.nnn.nnn (where "n" equals a number):

Now that you understand the history of IP address classes and how they're divided on byte boundaries, you're ready to grasp slash notation, a shorthand way of expressing network information. A Class A network address uses one byte, or eight bits, for the network address portion. To indicate this explicitly, administrators write /8, appended to the end of an IP address. When you see 126.10.11.12/8, you know it's a Class A address. A Class B uses two bytes, or 16 bits, written as /16. Class C appears as /24 (three eight-bit bytes). The important concept here is that in slash notation, the number after the slash tells you how many bits of an IP address is the network portion of the address. (You can readily calculate how many bits that leaves for the host portion of the address by subtracting the number of network bits from 32, the total number of bits in an IP address.)

How the Internet lost its class

When I left off my history of the Internet a few paragraphs ago, we had just seen that ARIN was running out of Class B network addresses. The Internet Engineering Task Force (IETF) eventually came up with a workaround. Instead of the original, rigid, network class splits on full-byte boundaries (/8, /16, and /24), they allowed Classless Internet Domain Routing (CIDR, pronounced like 'cider'). This birthed the modern way of talking about IP addresses, which ignores classes, and uses network notation instead. So if an organization wanted enough IP address space for one thousand systems, ARIN's choices were no longer limited to dispensing either four Class Cs (/24) or a Class B (/16) network. They could assign, for example, a /22. Twenty-two bits of the IP address would be used as the network address, and the remaining 10 bits would be used for host addresses, which would permit 1,024 addresses.

Another problem with IP addressing had surfaced in the mid-eighties. Some organizations wanted multiple network addresses, one for each network within their organization. Instead of giving out multiple network addresses, RFC 950 recommends the use of subnetworking, that is, using some portion of the IP address to indicate internal network addresses.

The problem with both using a CIDR (classless) network address, and splitting a network into subnetworks (abbreviated as subnets), was that TCP/IP was designed with the rigid Class system in mind. Certain addresses were assumed to have one, two or three bytes of network address. Specifically:

CIDR throws those assumptions away. In fact, this entire discussion of "classful" IP addresses is only necessary because of its long legacy on the Internet (and to help you understand Internet veterans who often use the old terminology). The problem was, in order to split IP addresses on any bit you wanted, there had to be some way of defining where that split in the IP address occurred. The answer for how to define subnets: use a subnet mask. It's a very important networking concept, and it's where we'll resume our discussion in "Understanding IPv4 Subnetting, Part 2".