We have seen why it is important to identify hosts and networks uniquely to be able to deliver packets reliably. Depending on the scale, there are three major ways of doing this; we will discuss each of those in this section. The end to end process of IP routing will be discussed in the next section. One interesting fact to note is that for each of these addressing modes, one or more addresses are reserved for special use. Often, these are marked by a known set of bits being on or off in a known pattern:
- Ethernet address: This is also known as a Media Access Control (MAC) address. It is a 48-bit long unique identifier assigned to a network device (usually stored on the card) that is used to identify it in a network segment. Usually, these are programmed by the network card manufacturer, but all modern OS's allow one to modify it. The standard way of writing Ethernet addresses are in six groups of two hexadecimal digits (01-23-45-67-89-ab-cd-ef). Another common way is to use a colon to separate the digits (01:23:45:67:89:ab:cd:ef). A few special sequences of bits are reserved for addressing special cases: the sender can request that an Ethernet frame should be received by all hosts in that segment by setting the least significant bit of the first octet to 1; this is called multicasting. If that particular bit is set to 0, the frame should be delivered to only one receiver. Today, these are used widely with Ethernet and Wi-Fi.
- IP address: This is an address assigned to each device in an IP network. The original IP address standard (IPv4) defined 32-bit addresses in 1980. However, by 1995, it was obvious that the total number of available addresses on the internet is not enough to cover all devices. This led to the development of IPv6, which expanded the address space to 128 bits. The standard way of dealing with a group of IP addresses is using the CIDR notation, for example, 192.168.100.1/26 (IPv4). The decimal number after the slash counts the number of leading 1s in the network mask. Thus, in this particular case, there are 2^(32-26) = 64 addresses in the network starting from 192.168.100.0 to 192.168.100.63. The Internet Assigned Numbers Authority (IANA) assigns blocks of publicly routable IP addresses to organizations. A number of IPv4 and v6 addresses are reserved for various purposes like addressing in private networks and so on. In a home network (which will always use special private range addresses), these are assigned by the Dynamic Host Configuration Protocol (DHCP) by the Wi-Fi router.
- Autonomous system number: This is a 32-bit number used to uniquely identify autonomous systems. Like IP addresses, these are assigned and maintained by the IANA.
Apart from these, communication between hosts often uses a port number to distinguish between processes. When the OS allocates a specific port to a process, it updates its database of the mapping between process identifier and port number. Thus, when it receives incoming packets on that port, it knows what process to deliver those packets to. In case the process has exited by that time, the OS will drop the packets and in the case of TCP, initiate closing of the connection. In the subsequent sections, we will see how TCP works in practice.
A range of port numbers between 0 and 1024 are reserved for common services by the OS. Other applications are free to request any port above 1024.