Essay by   •  August 29, 2010  •  Coursework  •  579 Words (3 Pages)  •  1,364 Views

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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

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" + 14 net bits + 14 host bits, hosts 128.1.0.0 to 191.254.0.0 Net IDs: 10 00000000000000 to 10 11111111111111

*Class C: "110" + 21 net bits(=24) + 8 host bits, hosts 192.0.1.0 to 223.255.254 *Class D: "1110" + 28 bits for multicast addresses (reserved), hosts 224.0.0.0

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