power of two 2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0
value in decimal 128 64 32 16 8 4 2 1
bits "borrowed" 1 2 3 4 5 6 7 8
netmask octet 128 192 224 240 248 252 254 255
What does it all mean?
If I have a 1 in that space in binary, it represents that
power of two, or that value, in decimal. for example:
octet = 00100000 = 2^5 = 32
octet = 00100100 = (2^5 + 2^2) = (32 + 4) = 36
octet = 10000001 = (2^7 + 2^0) = (128 + 1) = 129
If I have this octet in my netmask, this many bits are "borrowed"
by the network part of the address. for example:
mask = 255.255.240.0 = 11111111.11111111.11110000.00000000
four bits borrowed \__/
mask = 255.192.0.0 = 11111111.11000000.00000000.00000000
two bits borrowed \/
mask = 255.255.225.252 = 11111111.11111111.11111111.11111100
six bits borrowed \____/
the number of hosts = 2^n, where n is the number of host bits.
addresses are divided into a NETWORK part and a HOST part.
for a given IP address and mask, for example:
ip 146.115.138.66
mask 255.255.255.0
NETWORK PART ONLY + HOST PART ONLY = ADDRESS
ip address actual 146.115.138 + actual .66 = 146.115.138.66
network actual 146.115.138 + all zeros .0 = 146.115.138.0
first host actual 146.115.138 + one in last bit .1 = 146.115.138.1
broadcast actual 146.115.138 + all ones .255 = 146.115.138.255
last host actual 146.115.138 + ones except last .254 = 146.115.138.254
netMASK all ones 255.255.255 + all zeros .0 = 255.255.255.0
CAUTION: When he asks for the HOST PART ONLY, don't
give the WHOLE ADDRESS! Use only column two above!!
another example, this one supersized:
ip 202.45.83.105 (83=01010011 in binary)
mask 255.255.240.0 (4 bits borrowed for network)
NETWORK PART ONLY + HOST PART ONLY = ADDRESS
ip address actual 202.45.80 + actual 3.105 = 202.45.83.105
network actual 202.45.80 + all zeros 0.0 = 202.45.80.0
first host actual 202.45.80 + one in last bit 0.1 = 202.45.80.1
broadcast actual 202.45.80 + all ones 15.255 = 202.45.95.255
last host actual 202.45.80 + ones except last 15.254 = 202.45.95.254
netMASK all ones 255.255.240 + all zeros 0.0 = 255.255.240.0
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This page last revised Tue Jul 17 21:28:33 EDT 2001 by leaf