This is intended as a basic guide to how IP networking works at the basic level; traditionally such guides are for “dummies” but a lack of knowledge doesn’t make someone dumb but ignorant and being ignorant of one small esoteric part of computing is no crime. On the other hand, fixing that ignorance can help solve certain networking issues or at the very least make those domestic router settings make some sort of sense.
What is IP?
Whether we are talking about IPv4 (192.0.2.98/24) or IPv6 (2001:db8:1:1::1/64), these are both at a superficial level both “Internet Protocols”. Networks require some kind of physical networking layer underneath the IP layer – most commonly Ethernet for wired connections and WiFi for wireless connections, but it can run on many other networks – FDDI (historical), Fiber Channel, or InfiniBand. IP works above that level.
What made IP “special” for the time was the word internet; we think of this today as a world-wide network (“The Internet”) but back in the 1970s, the internet part of IP was about connecting multiple networks together with a gateway. Whilst network gateways existed before IP (and indeed after), they translated one network protocol into another and not infrequently were application specific (i.e. perhaps only allowed email through) whereas IP was build from the ground up to allow network traffic to traverse multiple network gateways.
There’s a whole lot of detail we could get into about the “protocol” bit of IP, but an an early stage all we need to know is that the packet of data contains an IP header which amongst other things specifies the source address, destination address, and a hint of what’s inside.
An IP address (whether a source address or a destination address) is either a 32-bit integer (for IPv4) or a 128-bit integer (for IPv6). Which is the technical way of saying they’re just numbers; although we’re used to seeing (and using) a standardised representation of that number. Some tools will convert the representation of the address or will use the actual number :-
» ping 3221226082 PING 3221226082 (192.0.2.98) 56(84) bytes of data.
So the usual representation of an IPv4 address is a “dotted quad” – four numbers between 0-255 which when converted to binary and concatenated make up the real network address.
On the other hand, IPv6 is somewhat longer and uses groups of four hexadecimal digits separated with colons (“:”) as in:
Although it is usual to compress that with two simple rules – firstly a number of sequences of all zeros can be replaced once with “::” :-
Secondly, leading zeros within each ‘group’ can be dropped :-
Even shortened, IPv6 addresses are somewhat more complex than IPv4 addresses, but that’s why we have the DNS; in fact those used to using IPv4 addresses without the DNS should bear in mind that IPv4 addresses are also too complex for normal people to get right (and are subject to typos).
Whether you are assigned a public IPv6 address (2001:db8:cafe/48) or pick a private address from RFC1918 (10/8) you have a “choice” – you can either use the entire network range for one huge network, or you can sub-divide it into smaller subnets. Most home users will go with the first option but for most organisations of a reasonable size, the later is not just preferred but essential.
Whether you further divide a network into subnets or not, your computer still needs to know whether to send packets directly to the destination or whether to route those packets via a gateway or a router. This is done with a netmask that defines the network part of the address (and the host part of the address). If the network address of the destination matches the network address of the source, then the packets can be sent directly. Otherwise they’re sent via the gateway.
IPv6 addresses work in the same way except the addresses are longer.
Although netmasks are historically given as dotted quads making them look a bit like IPv4 addresses, it is becoming increasingly common to use a more compact method which is less error prone. The netmask is instead specified as the number of bits that the netmask covers – 192.0.2.98/24 rather than 192.0.2.98/255.255.255.0. As for IPv6, the same applies although “/64” is very often assumed – the default size for an IPv6 network is very much more strongly encouraged than for an IPv4 network (although it isn’t compulsory).
The Gateway Or The Router
Gateway or router? Well both – from the perspective of an ordinary host it’s a gateway to other networks; from the perspective of the gateway itself, it is a router connected to multiple networks (domestically often just two) and forwards packets on behalf of other computers.
In essence there is very little difference between a router and an ordinary machine except that the ordinary machine isn’t configured to forward packets, and it is usually configured with just a default gateway (sometimes called a gateway of last resort). Well and the route for the network it is connected to.
Both contain a routing table (or more than one) in the operating system kernel which basically consists of a set of network addresses and destinations (where to forward the packets to). In the case of your usual domestic router that usually consists of a route to your home network, a route to the ISP’s network, and a default route pointing at the ISP’s router.
When a machine wants to send (or forward) a packet to a destination, it picks the closest match in the routing table, and uses that as a intermediate destination to forward the packet to. Your machine operates this way; as does the core Internet routers (although they have slightly larger routing tables).
Some routers (probably a minority) are rather more complex of course. If you have heard of routing software such as BGP, OSPF, or IS-IS, then you have heard of software that distributes routing information. The larger Internet uses BGP to distribute routing information to add to routing tables around the world.
The description of routing so far has been rather hierarchical – your computer forwards to a default gateway, and it in turn forwards to your ISP’s default gateway. Which is a bit unfortunate as Internet routing doesn’t really work this way – there are alternate routes so if one router goes “bang” traffic can still reach the destination.