Practical TCP/IP and Ethernet Networking- P22: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | Internet layer protocols 87 an IPv4 network address is assigned to an organization it is done in the form of a 32-bit network address and a corresponding 32-bit mask. The ones in the mask cover the NetID and the zeros cover the HostID. The ones always run contiguously from the left and are called the prefix. An address of with a mask of 11111111111111111111111111000000 ones in the first 26 positions would therefore be said to have a prefix of 26 and would be written as 26. The subnet mask in this case would be . Note that this address in terms of the conventional classification would have been regarded as a class C address and hence would have been assigned a prefix of 24 subnet mask with ones in the first 24 positions by default. Classless inter-domain routing CIDR A second problem with the fashion in which the IP addresses were allocated by the Network Information Center NIC was the fact that it was done more or less at random and that each address had to be advertised individually in the Internet routing tables. Consider for example the case of following 4 private traditional class C networks each one with its own contiguous block of 256 254 useable addresses Network A IP addresses Network B IP addresses Network C IP addresses Network D IP addresses Assuming that there are no reserved addresses then the concentrating router at the ISP would have to advertise 4 x 256 1024 separate network addresses. In a real life situation the ISP s router would have to advertise tens of thousands of addresses. It would also be seeing hundreds of thousands if not millions of addresses advertised by the routers of other ISPs across the globe. In the early nineties the situation was so serious it was expected that by 1994 the routers on the Internet would no longer be able to cope with
