1) understand the setup for CIDR i.g. 4 groups of 8 bits equaling 32 (example)
11111111.11111111.11111111.11111111 = 32 bits (4 groups of 8 separated by a "." )
2) total number in each group = 255
11111111 = 255
WTF? each group is easily calculated with these numbers(blocks)!
128+64+32+16+8+4+2+1 = 255 (each bit(block) has a number, add all of them to get 255)
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3) Calculate the CIDR netmask of /30
a) count out 30 bits of 1 one(s) and the rest are zero(s) equaling 32 bit(s)
11111111.11111111.11111111.11111100
30 one(s) and 2 zero(s) which equal = 32 bits
4) seperate and count each group:
11111111.11111111.11111111.11111100
group 1 = 11111111 = 255
128+64+32+16+8+4+2+1 = 255
group 2 = 11111111 = 255
128+64+32+16+8+4+2+1 = 255
group 3 = 11111111 = 255
128+64+32+16+8+4+2+1 = 255
group 4 = 11111100 = 252
128+64+32+16+8+4 = 252
NOTE: for each zero in a group remove a bit(block) starting from 1(right to left)
128+64+32+16+8+4+2+1 = 255
the calculated CIDR netmask of /30 = 255.255.255.252
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diagram of CIDR:
Netmask Netmask (binary) CIDR Notes
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255.255.255.255 11111111.11111111.11111111.11111111 /32 Host (single addr)
255.255.255.254 11111111.11111111.11111111.11111110 /31 Unuseable
255.255.255.252 11111111.11111111.11111111.11111100 /30 2 useable
255.255.255.248 11111111.11111111.11111111.11111000 /29 6 useable
255.255.255.240 11111111.11111111.11111111.11110000 /28 14 useable
255.255.255.224 11111111.11111111.11111111.11100000 /27 30 useable
255.255.255.192 11111111.11111111.11111111.11000000 /26 62 useable
255.255.255.128 11111111.11111111.11111111.10000000 /25 126 useable
255.255.255.0 11111111.11111111.11111111.00000000 /24 "Class C" 254 useable
255.255.254.0 11111111.11111111.11111110.00000000 /23 2 Class C's
255.255.252.0 11111111.11111111.11111100.00000000 /22 4 Class C's
255.255.248.0 11111111.11111111.11111000.00000000 /21 8 Class C's
255.255.240.0 11111111.11111111.11110000.00000000 /20 16 Class C's
255.255.224.0 11111111.11111111.11100000.00000000 /19 32 Class C's
255.255.192.0 11111111.11111111.11000000.00000000 /18 64 Class C's
255.255.128.0 11111111.11111111.10000000.00000000 /17 128 Class C's
255.255.0.0 11111111.11111111.00000000.00000000 /16 "Class B"
255.254.0.0 11111111.11111110.00000000.00000000 /15 2 Class B's
255.252.0.0 11111111.11111100.00000000.00000000 /14 4 Class B's
255.248.0.0 11111111.11111000.00000000.00000000 /13 8 Class B's
255.240.0.0 11111111.11110000.00000000.00000000 /12 16 Class B's
255.224.0.0 11111111.11100000.00000000.00000000 /11 32 Class B's
255.192.0.0 11111111.11000000.00000000.00000000 /10 64 Class B's
255.128.0.0 11111111.10000000.00000000.00000000 /9 128 Class B's
255.0.0.0 11111111.00000000.00000000.00000000 /8 "Class A"
254.0.0.0 11111110.00000000.00000000.00000000 /7
252.0.0.0 11111100.00000000.00000000.00000000 /6
248.0.0.0 11111000.00000000.00000000.00000000 /5
240.0.0.0 11110000.00000000.00000000.00000000 /4
224.0.0.0 11100000.00000000.00000000.00000000 /3
192.0.0.0 11000000.00000000.00000000.00000000 /2
128.0.0.0 10000000.00000000.00000000.00000000 /1
0.0.0.0 00000000.00000000.00000000.00000000 /0 IP space
Note: I dont want to plagerise so i have to give most of the credit to the people in this forum who discussed on
how to get this calculated. I just(hopefully) made sense of the confusion!
http://www.techexams.net/forums/ccna-ccent/38980-help-figuring-out-cidr-notation-calculations.html
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