The most common type of local network is called Ethernet. This word actually describes a collection of different media and signal types. All these different technologies are united in the sense that they are organized logically around the idea of packets. A packet is an ordered collection of ones and zeros that have a particular significance -- if you will, an electrical or optical signal with a beginning, some content, and an end.
Each packet contains an address for the local machine that is its immediate destination. That address is called the Ethernet address, or more often, the MAC (Media Access Control) address. The MAC address is a 48-bit binary number. In writing the number, we don't want to use the space and time required to put down each of the ones and zeros, so it is organized into six octets. Typically, each octet is written using hexadecimal notation, and the octets are separated by colons. Thus, MAC addresses are typically written in forms such as 08:00:A0:B1:18:03 or 00:05:15:64:68:F0. We see that the 48-bit address is written as twelve hexadecimal digits. As we noted above, each hex digit represents four bits of information.
It is worth noting that MAC addresses are purchased in blocks of 16,777,216 by the manufacturers of Ethernet interfaces. This means that the first three octets in the MAC address identify the manufacturer of an Ethernet card, while the last three octets are simply a number from within that sequence. Thus, each MAC address uniquely identifies an Ethernet interface ("network card").
Every Ethernet packet contains either a MAC address of its immediate destination, or an address that indicates that it is intended to be received by every computer on the local network. In this way, the receiving computer understands which of the packets on the network are intended for it, and which are not. Each Ethernet packet also contains the address of the computer that sent it, so that if a reply is needed, it may be sent.
There are many other types of computer networks, but we do not need to consider them or the details of Ethernet networks. Our point here was simply to show the way that the physical network of cables and appliances fits together with the logical network of ones and zeros, expressed using hex notation.