Twenty years ago, there were many competing networking protocols. Today, one protocol is overwhelmingly common—the Internet Protocol. It comes in two versions—IPv4 and IPv6. IPv4 is completely ubiquitous and deployed everywhere. If you're deploying network code today, you must support IPv4 or risk that a significant portion of your users won't be able to connect.
IPv4 uses 32-bit addresses, which limits it to addressing no more than 232 or 4,294,967,296 systems. However, these 4.3 billion addresses were not initially assigned efficiently, and now many Internet Service Providers (ISPs) are forced to ration IPv4 addresses.
IPv6 was designed to replace IPv4 and has been standardized by the Internet Engineering Task Force (IETF) since 1998. It uses a 128-bit address, which allows it to address a theoretical 2128 = 340,282,366,920,938,463,463,374,607,431,768,211,456, or about a 3.4 x 1038 addresses.
Today, every major desktop and smartphone operating system supports both IPv4 and IPv6 in what is called a dual-stack configuration. However, many applications, servers, and networks are still only configured to use IPv4. From a practical standpoint, this means that you need to support IPv4 in order to access much of the internet. However, you should also support IPv6 to be future-proof and to help the world to transition away from IPv4.
All Internet Protocol traffic routes to an address. This is similar to how phone calls must be dialed to phone numbers. IPv4 addresses are 32 bits long. They are commonly divided into four 8-bit sections. Each section is displayed as a decimal number between 0 and 255 inclusive and is delineated by a period.
Here are some examples of IPv4 addresses:
0.0.0.0127.0.0.110.0.0.0172.16.0.5192.168.0.1192.168.50.1255.255.255.255
A special address, called the loopback address, is reserved at 127.0.0.1. This address essentially means establish a connection to myself. Operating systems short-circuit this address so that packets to it never enter the network but instead stay local on the originating system.
IPv4 reserves some address ranges for private use. If you're using IPv4 through a router/NAT, then you are likely using an IP address in one of these ranges. These reserved private ranges are as follows:
10.0.0.0to10.255.255.255172.16.0.0to172.31.255.255192.168.0.0to192.168.255.255
The concept of IP address ranges is a useful one that comes up many times in networking. It's probably not surprising then that there is a shorthand notation for writing them. Using Classless Inter-Domain Routing (CIDR) notation, we can write the three previous address ranges as follows:
10.0.0.0/8172.16.0.0/12192.168.0.0/16
CIDR notation works by specifying the number of bits that are fixed. For example, 10.0.0.0/8 specifies that the first eight bits of the 10.0.0.0 address are fixed, the first eight bits being just the first 10. part; the remaining 0.0.0 part of the address can be anything and still be on the 10.0.0.0/8 block. Therefore, 10.0.0.0/8 encompasses 10.0.0.0 through 10.255.255.255.
IPv6 addresses are 128 bits long. They are written as eight groups of four hexadecimal characters delineated by colons. A hexadecimal character can be from 0-9 or from a-f. Here are some examples of IPv6 addresses:
0000:0000:0000:0000:0000:0000:0000:00012001:0db8:0000:0000:0000:ff00:0042:8329fe80:0000:0000:0000:75f4:ac69:5fa7:67f9ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff
Note that the standard is to use lowercase letters in IPv6 addresses. This is in contrast to many other uses of hexadecimal in computers.
There are a couple of rules for shortening IPv6 addresses to make them easier. Rule 1 allows for the leading zeros in each section to be omitted (for example, 0db8 = db8). Rule 2 allows for consecutive sections of zeros to be replaced with a double colon (::). Rule 2 may only be used once in each address; otherwise, the address would be ambiguous.
Applying both rules, the preceding addresses can be shortened as follows:
::12001:db8::ff00:42:8329fe80::75f4:ac69:5fa7:67f9ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff
Like IPv4, IPv6 also has a loopback address. It is ::1.
Dual-stack implementations also recognize a special class of IPv6 address that map directly to an IPv4 address. These reserved addresses start with 80 zero bits, and then by 16 one bits, followed by the 32-bit IPv4 address. Using CIDR notation, this block of address is ::ffff:0:0/96.
These mapped addresses are commonly written with the first 96 bits in IPv6 format followed by the remaining 32 bits in IPv4 format. Here are some examples:
IPv6 Address | Mapped IPv4 Address |
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You may also run into IPv6 site-local addresses. These site-local addresses are in the fec0::/10range and are for use on private local networks. Site-local addresses have now been deprecated and should not be used for new networks, but many existing implementations still use them.
Another address type that you should be familiar with are link-local addresses. Link-local addresses are usable only on the local link. Routers never forward packets from these addresses. They are useful for a system to accesses auto-configuration functions before having an assigned IP address. Link-local addresses are in the IPv4 169.254.0.0/16 address block or the IPv6 fe80::/10 address block.
It should be noted the IPv6 introduces many additional features over IPv4 besides just a greatly expanded address range. IPv6 addresses have new attributes, such as scope and lifetime, and it is normal for IPv6 network interfaces to have multiple IPv6 addresses. IPv6 addresses are used and managed differently than IPv4 addresses.
Regardless of these differences, in this book, we strive to write code that works well for both IPv4 and IPv6.
If you think that IPv4 addresses are difficult to memorize, and IPv6 addresses impossible, then you are not alone. Luckily, we have a system to assign names to specific addresses.