Subnet Masking and Addressing

Ok, this explains subnet addressing which is useful if you run a scanner, a firewall, a router or anything else that is bound to IP subnet addressing. Note that this only describes IPv4 subnets. Reading binary values

Normally, you read binary numbers bytewise (8 bit wise). Start at the last bit, bit 0. If it is 1, add 2^0 to your number, else add 0. Then the next bit, bit 1, If it is 1, add 2^1 (2) to your number, If bit 3 is 1 add 2^2 (4) to your number, if bit 4 is 1 add 2^3 (8) to your number ... if bit 8 is 1 add 2^7 (128) to your number. You see, the base is always 2 because it can be either 0 or 1. Example 1: 10100100 = 2^7+0+2^5+0+0+0+2^2+0+0 = 164 Example 2: 11111111 = 2^7+2^6+2^5+2^4+2^3+2^2+2^1+2^0 = 255 Thats it! Now to subnet addressing.

When you state a host including a subnet (example: nmap), you do it like this: 1.2.3.4/24, where /24 is the subnet. Lets have a look at what this means: an IP address is a 32 bit address. It is divided into 4 bytes (each 8 bits meaning they can be 0 to 255) in general notation:

00000001 00000010 00000011 00000100 = "1.2.3.4"

now, IP uses one part of this address to specify which Net it is on. Most of the time, this is a physical Net like an ethernet LAN that is linked to the internet. Nets that link to the internet get dedicated IPs for each of their hosts from the IANA.org. /24 means that the first 24 bits are the Net address and the remaining 8 bits are the Host address. This looks like this: Net: 000000010000001000000011 Host: 00000100 Meaning, we are on the net 1.2.3.0 (0 used as a wildcard here) and on the host 4 of 256.

SUBNET MASK: In this case, the subnet mask would be 255.255.255.0. A subnet mask is created simply by filling all NET address bits with 1 and the HOST bits with 0. (11111111 = 255). There are 4 "Classes" on the Internet, which are the standard Subnets. *Class A: "0" + 7 net bits + 24 host bits, hosts 0.0.0.0 to 126.255.255.255 Net IDs: 0 0000000 to 0 1111111 (which is 127 => 127.0.0.0 reserved for local loopback)

*Class B: "10"