What are netblocks, and how are they useful?
An IP address is divided into two parts, the address of the network, and the address of the machine. Which is which depends on what the first number of the IP address is:
Class A
A class A network uses the first number as the network address, so you can have 16.7 million (2^24) nodes in that network. The network address must also be between 1 and 126. (127 is loopback). For example, net 38 is owned by psi.net.
Class B
A class B network uses the first 2 numbers as a network address which makes for 65,535 (2^16) possible nodes. Class B networks range between 128.0 and 191.255. For example, 153.34 is owned by uu.net.
Class C
Class C networks use the first 3 numbers as a network address with 256 possible nodes. Class Cs range between 192.0.0 and 223.255.255. For example, 199.234.240 is owned by Oasis Telecommunications.
Class D and Class E
Class D is for networks 224 to 239.255.255.255. Class E is for networks 240 to 255.255.255.255. Class D is for multicast messages and class E is reserved for experimentation and development. If you see one of these IP addresses in a header, you can be quite certain that the header has been forged. (Or there is a serious configuration problem somewhere.)
