There are a couple of big ideas to consider from the first few chapters of a book titled “Linked” that help to understand the geography of networks. The first is the idea, from Paul Erdős and Alfred Rényi, is the random network. In a random network, links are placed randomly. There’s no particular reason behind the connection between any two nodes. In a random network, all nodes will have the same number of links, more or less. This means that a random network can be shown as a distributed network, as Figure 1 below shows.
When you want to send a message across this network, it helps to know something about the distance across the network. This isn’t a physical distance, but a distance measured in links. Any network can be measured in terms of the number of hops it takes to get a message from one node to another. In the Figure 1, you can see a distributed network of a few dozen nodes. If you want to get a message from node A on one side of the network to node Z on the other side, the message has to go to the nearest node that’s closer to B than A itself. In this diagram, B is closer to Z than A. Likewise, C is closer than B, so the message goes from B to C. That process continues until the message gets to Z.
Chapter 4 of the book “Linked” introduces Mark Granovetter’s notion of small worlds networks. These networks are not so random, as some connections are strong ties, and some are weak. Figure 2 below shows a network in which each node is connected strongly to its nearest four neighbors. The distance across such a small worlds network can get relatively long, but it only takes one or two ties across the network to shrink the distance considerably. Duncan Watts and Steve Strogatz, building on Granovetter’s ideas, developed the notion of clustering on networks, in which one subnet might have strong ties to each other, and weak ties across the larger network. These weak ties perform the important function of connecting the larger network. As an example of what weak ties can do, Granovetter showed through his research how many people find jobs not through their strong ties to family or friends, but through weak ties to friends of friends, acquaintances, and so forth. In internet terms, many connections between local networks might be thought of as weak ties.
The internet’s structure is as follows: end devices are connected to your router, which is connected to other routers, which are connected to other routers, and so forth. Whether you’re on a home network, academic network, business network, or mobile network, It’s routers all the way down. How are these machines addressed?
Recall the Open Systems Interconnect model of networks, shown in Table 1. Addressing is handled on the second and third layers.
|Table 1. The Open Systems Interconnect (OSI) Network Model|
|Application||What are you doing with the data transmitted or received?|
|Presentation||How is the data formatted and/or encrypted?|
|Session||How do you say hello and goodbye?|
|Transport||How are you sending data packets?
Should the receiver acknowledge them?
|Network||What’s your address on this network?
How do you connect to other networks?
|Datalink||What’s the physical address of your device, and who’s talking?|
|Physical||What physical medium are you transmitting on?|
On the data link layer, each network interface, such as the WiFi radio or Ethernet jack in your computer, gets a unique hardware address called the Media Access Control address. One device may have multiple MAC addresses if it has multiple network interfaces. For example, a laptop that has a WiFi radio and an Ethernet jack has two network interfaces, and therefore two MAC addresses. Manufacturers of IP-compatible devices license blocks of addresses from the Institute of Electrical and Electronics Engineers (IEEE), which registers MAC addresses. Each address is six bytes long and identifies a network interface of a device. The first three bytes of a MAC address are called the Organizational Unique Identifier (OUI). These identify companies which make devices that use MAC addresses. OUIs are registered by the IEEE. The second three bytes of a MAC address uniquely identify your network interface. The field below shows a typical MAC address.
final three bytes are
your unique device
first three bytes are the
IP Address space is administered by theInternet Assigned Names Authority (IANA), throughPublic Technical Identifiers (PTI), incorporated in 2016, an affiliate of the Internet Corporation For Assigned Names and Numbers (ICANN). IANA manages address space through its Regional Internet Registries (RIRs). Internet Service Providers license addresses from the RIRs.
|Table 2. Regional Internet Registies|
|ARIN||Canada, USA, and some Caribbean Islands|
|LACNIC||Latin America and some Caribbean Islands|
|RIPE NCC||Europe, the Middle East, and Central Asia|
The IANA Regional Internet Registries (RIRs). Table and figure from IANA.
Each address for IPv4 is four bytes long and identifies a device on a network. IPv4 space is divided into public IP addresses and private IP addresses. There are also blocks reserved for multicast use, and for future use.
IP Addresses are divided into four bytes, each representing a range of addresses. The first byte determines the largest block of addresses in a given network, and the last determines the smallest block of addresses. For example, a network defined as 128.xxx.xxx.xxx could have up to 224 possible devices on it, while 128.122.6.xxx could only have up to 28, or 256 devices on it.
Private address ranges are ranges of IP addresses designed to be used for local network addresses only. They mean nothing outside the domain of a given router. For example, your home router might have the address 10.0.1.1, and it might assign your computers the addresses 10.0.1.2, 10.0.1.3, 10.0.1.4, and so forth. But these numbers are not reachable by a device outside your router’s network. Common private IP address ranges are:
- 10.0.0.0, a 24-bit block of addresses
- 172.16.0.0, a 20-bit block of addresses
- 192.168.0.0, a 16-bit block of addresses
On the network layer, routers use Address Resolution Protocol (ARP) to associate MAC addresses with available IP addresses. When a new device connects to a network, it announces its MAC address and requests an IP address. Depending on the rules of the network, the router either grants an IP address or denies it. Many routers default to permissive addressing, meaning that they will assign an IP address to any legitimate ARP request. However, most enterprise networks, such as academic networks and business networks, will maintain tables of registered MAC addresses that can be granted an IP address. Any addresses not in that table will not be assigned an IP address.
Because an ISP licenses a chunk of IP address space, that doesn’t mean they can provide physical access to the network. Conversely, because a network provider has fiber to your building, it doesn’t mean they can provide you with IP addresses. The two often go hand-in-hand, but not always.
In order to connect to a network, a private network has to have a public gateway. Its router performs this function, and therefore has two network addresses, one public and one private. Figure 5 shows a typical home setup, in which the router has both a public and a private IP address, and assigns private IP addresses to the laptop and tablet connecting to it. The router’s public IP address is in the range of the ISP’s subnet.
One large network may combine several smaller ones. A large network can combine a combination of public and private networks. Ultimately every device with a private address will be “represented” to the rest of the internet by the first router above it with a public address. Figure 6 shows this in action. The central router has the address 128.122.x.x, and can therefore form the largest network. Each router attached to this router can form one or more public or private networks of its own, using addresses within the central router’s range.
Finally, Autonomous Systems are networks of networks. They are joined using the Border Gateway Protocol (BGP). Autonomous System routers maintain routing tables not only for their own network, but for the networks to which they connect. They tell each other how traffic should be routed. When they fail, major traffic problems occur. Major internet service providers maintain these kinds of routers. Figure 7 below shows a network of autonomous systems, featuring some actual AS numbers that you can look up.
- AS12: NYU-DOMAIN
- AS19905: NEUSTAR-AS6
- AS224: RIPE NCC ASN block
- AT&T: AT&T Internet Services