Addressing in networks

The Internet is made up of billions of participants who simultaneously communicate with one another. If a LAN is connected to the Internet, it becomes a part of the Internet.

For all LANs to exchange data with each other, they must be distinguishable.

In 1981, the Internet Protocol (IP) was introduced. Each system on the Internet possesses a unique 32-bit address.

InterNIC (Inter Network Information Centre) is the central international issuing office. Its German counterpart is the DENIC.

There are different address classes for different numbers of IP addresses.

IP Addresses

Each source and destination of an IP packet must have an IP address. IP addresses have a length of 32 bits (4 bytes of 8 bits each).

IP addresses cannot be chosen freely. They are made up of a string of 4 three-digit decimal numbers, with each pair separated by a full stop (dotted decimal notation).

Example: 192.168.1.19

The first section (192.168.1) indicates the computer’s network (network ID). The last section (.19) identifies the computer (host ID).

The subnet mask serves as a precise distinction between network ID and host ID.

Example: 255.255.255.0

The above example of a subnet mask shows that the first three bytes of the IP address belong to the network ID and that the last byte belongs to the host ID. This allows it to be classified into three classes (A, B and C), which is explained in the section below.

IP address classes

Network ID and host ID may have different lengths. Neither network ID nor host ID can contain exclusively zeros or ones.

In the classes A – C, the lowest and the highest host ID are of particular importance:

• The lowest host ID 0 is not permitted (all bits of the host ID are 0).
• The highest available host ID is the so-called broadcast address, with which all hosts can be reached. This also can’t be used as an address.

Class A (subnet mask: 255.0.0.0)

24 bits remain for the host ID. Consequently, 16.777.214 different host addresses are possible.

Class B (subnet mask: 255.255.0.0)

16 bits remain for the host ID. Consequently, 65,534 host addresses are possible.

Class C (subnet mask: 255.255.255.0)

8 bits remain for the host ID. Consequently, 254 host addresses are possible.

Ports

Computers within a network can be addressed using IP addresses. But how does the computer know which program the data relates to? As we know, checking your emails while browsing is no problem at all. Logically, this means there must be something that allocates data packages to their respective software. This is accomplished by ports.

But what does that mean?

Example: letterbox

In this case the IP or computer name corresponds to the postal address. The port is the corresponding apartment.

Important (well-known) ports

Ports in the range of 0 to 1023 are reserved for special applications and services (well-known ports). Registered ports for applications can be found in the range of 1024 to 49151.

A server service requires a stable and well-known port, so that it can be addressed by the client application. However, the client application can use any port in the range of 49152 to 65535, allocated upon request for a given period (dynamic port).

Applicable to dynamic ports:

The application and the server negotiate a port to be used for further communication, which makes it possible to start and use the application multiple times. The user usually does not notice the process since all of the above happens in the protocols in the background.

Well-known ports and ports you should have at least heard of:

• 20/21 FTP (file transfer, port 20 defines the port for the data channel, port 21 defines the port for the control channel)
• 23 Telnet (remote administrations)
• 25 SMTP (email)
• 80 HTTP (World Wide Web)
• 443 HTTPS (encrypted WWW)
• 161/162 SNMP (network management)