Lindy MC5-IP manual Appendix 6 Addresses, masks and ports, IP addresses, Net masks

Appendix 6 – Addresses, masks and ports

IP address, network masks and ports are all closely linked in the quest for one device to find another across disparate network links.

IP addresses

As a rough analogy, consider how you use the telephone system. The phone number for LINDY in the UK is 0044 (0)1642 754000. This number consists of three distinct parts:

•0044 connects from another country to the UK

•(0)1642 selects the main telephone exchange in the Thornaby area of Stockton-on-Tees, and

•754000 is the unique code for LINDY within Thornaby.

The important parts of the whole number depend on where you are. If you were based in the same local area as LINDY, there would be no point in dialling out of the UK, or even out of the area. The only part of the whole number that you are interested in is the final part: 754000.

In a similar way to the various parts of the telephone number, the four sections (or Octets) of every IP address have different meanings or “weights”. Consider the following typical IP address:

192.168.142.154

192 is the most global part of the number (akin to the 0044 of the phone number) and 154 is the most local (similar to the 754000 unique local code of the phone number).

When two network devices communicate with each other, they always “dial the whole number” regardless of their respective locations in a network. However, they still need to know whether the other device is local to them or not, and this is where the net mask comes into play.

Net masks

The net mask (or sub-net mask) informs a device as to its own position within a network. From this it can determine whether any other device is within the same local network or is situated further afield.

Taking the telephone number analogy given in the IP address section, in order to use the telephone system efficiently, it is vital for you to know your location relative to the person you are calling. In this way you avoid dialling unnecessary numbers.

When one network device needs to talk to another, the first thing that it will do is a quick calculation using its own IP address, the other device’s IP address and its own net mask. Suppose a device with address 192.168.142.154 and net mask 255.255.255.0 needed to communicate with a device at address 192.168.142.22. The sending device would perform several calculations:

1	Sending	Destination
192 168 142 154	device IP	device IP	192 168 142 22
	address	address
	Net mask
		3
192 168 142 000	Result		192 168 142 000
	2
192.168.142.000		= 192.168.142.000 ?		Answer: YES
			4	Address
				is local

1 The net mask is used to determine the local and global parts of the sender’s IP address. Where there is 255 in the mask, the corresponding address slips through, where there is a 0, it is blocked.

2 Where the net mask was 0, the corresponding part of the result is also zero - this section is now known to be the local part of the IP address.

3 The same process is carried out for the destination address, again using the sender’s net mask. Now the local parts of both addresses have been equalised to zero, because their values are not important in determining whether they are both in the same local network.

4 The results of the two net mask operations are now compared, if they match, the destination is local. If not, then the sender will still use the same full destination IP address but will also flag the message to go via the local network gateway and out into the wider world.

The reason for doing this? It makes the network, as a whole, much more efficient. If every message for every recipient was shoved straight out onto the Internet, the whole thing would grind to a halt within seconds. Net masks keep local traffic just that - local.

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