Netmasks and Netblock Sizes
Netblock Sizes, Netmasks, and Bit-sized Networks
The following table shows the relationship between IP network's block
sizes, classful interpretation, netmasks, "supernets" [bit-length], "cisco
wild bits" and the actual bit-sizes of the networks.
An IP address on its own does not communicate what size network block to
which it belongs. A netmask associated with an IP address describes
"masked" portions [the left-most bits 'blocked out' by the ones in the
netmask] of an IP address. These masked portions create the "network
part" of the address, commonly called the "prefix".
Block Size
The block size tells you how many addresses are within the
network. The "bottom" [all-zero] is reserved to refer to the network itself
and "top" [all-ones] is reserved as the broadcast address, so these are
unusable. therefore, any given netblock has two less usable addresses that the
literal block size.
Eg, a network 10.0.0.64 with a block size of four
has only two usable addresses, 10.0.0.65 and 10.0.0.66.
Netmask
The netmask indicates the final (or final two) octets
of what should be masked against the address to configure equipment or
define routes with the apropriately-sized network. All leading
[leftmost] octets in a netmask are ones.
Eg, a network 10.0.16.0 with a netmask of
252.0 would result in a network netmask pair of
10.0.16.0 255.255.252.0, while the previous example from block
size would result in 10.0.0.64 255.255.255.252.
Cisco "Wild Bits"
The cisco wild bits indicate the apropos value for the
final (or final two) octets of what is needed when creating a Cisco
router's access list for a certain network size. It is one less than
the block size, merely because given the network number [the "zero" or
"bottom" of the network], the "wild bits" define all the remaining
possible values within the network.
"Supernet"
The supernet notation indicates the literal bitsize of
a network. "Supernet" is actually a misnomer, stemming from the
perceived need to differentiate prefixes longer than 24-bits [subnets of
"Class C" blocks] from those shorter than 24-bits in traditional "Class
C space" (eg, the 207.* network), calling ther latter "supernets". The
number actually represents the "prefix length" - what part of an address
is the network-part.
Given that IPv4 addresses are 32-bits long, this is the most
common and easy way of describing a network. A network of one device (a
host route) is defined as easily as a network of two hundred fifty-six
devices: 10.0.99.99/32 is a host and 10.0.98.0/24 is a "C-sized block".
Raw Bits
The raw bits define the bits "within" the network. These
are the inverse of the supernet notation, and are a bitwise
representation of the blocksize. Lastly, this is the exponent for "2"
when describing the network mathematically.
Classful
The classful row merely gives a "backwards compatible"
reference. All classful interpretations [Class A, B, C, D and E
networks] of IPv4 space are archaic. This row is provided to give a
reference point when speaking to customers that do not yet have a firm
grasp of "supernet" notation.
Reference Table
Netmasks and Wild Bits refer to the RIGHTMOST octets.
block size 1 2 4 8 16 32 64 128 256
netmask 255 254 252 248 240 224 192 128 0
cisco wild bits 0 1 3 7 15 31 63 127 255
supernet /32 /31 /30 /29 /28 /27 /26 /25 /24
raw bits 0 1 2 3 4 5 6 7 8
Classful |------ Subnets of Class C addresses ------| Class C
block size 512 1024 2048 4096 8192 16384 32768
netmask 254.0 252.0 248.0 240.0 224.0 192.0 128.0
cisco wild bits 1.255 3.255 7.255 15.255 31.255 63.255 127.255
supernet /23 /22 /21 /20 /19 /18 /17
raw bits 9 10 11 12 13 14 15
Classful |--------- Subnets of Class B addresses --------|
block size 65536
netmask 0.0
cisco wild bits 255.255
supernet /16
raw bits 16
Classful Class B
Thank you Joe Provo for the above v4 document!
IPv6
2^64 = 18,446,744,073,709,551,616
Prefix /48 count /56 count Number of /64 Subnets Number of Hosts
/56 1 256 4,722,366,482,869,645,213,696 (2^72)
/55 2 512 9,444,732,965,739,290,427,392
/54 4 1,024 18,889,465,931,478,580,854,784
/53 8 2,048 37,778,931,862,957,161,709,568
/52 16 4,096 75,557,863,725,914,323,419,136
/51 32 8,192 151,115,727,451,828,646,838,272
/50 64 16,384 302,231,454,903,657,293,676,544
/49 128 32,768 604,462,909,807,314,587,353,088
/48 1 256 65,536 1,208,925,819,614,629,174,706,176 (2^80)
/47 2 512 131,072 2,417,851,639,229,258,349,412,352
/46 4 1,024 262,144 4,835,703,278,458,516,698,824,704
/45 8 2,048 524,288 9,671,406,556,917,033,397,649,408
/44 16 4,096 1,048,576 19,342,813,113,834,066,795,298,816
/43 32 8,192 2,097,152 38,685,626,227,668,133,590,597,632
/42 64 16,384 4,194,304 77,371,252,455,336,267,181,195,264
/41 128 32,768 8,388,608 154,742,504,910,672,534,362,390,528
/40 256 65,536 16,777,216 309,485,009,821,345,068,724,781,056
/39 512 131,072 33,554,432 618,970,019,642,690,137,449,562,112
/38 1,024 262,144 67,108,864 1,237,940,039,285,380,274,899,124,224
/37 2,048 524,288 134,217,728 2,475,880,078,570,760,549,798,248,448
/36 4,096 1,048,576 268,435,456 4,951,760,157,141,521,099,596,496,896
/35 8,192 2,097,152 536,870,912 9,903,520,314,283,042,199,192,993,792
/34 16,384 4,194,304 1,073,741,824 19,807,040,628,566,084,398,385,987,584
/33 32,768 8,388,608 2,147,483,648 39,614,081,257,132,168,796,771,975,168
/32 65,536 16,777,216 4,294,967,296 79,228,162,514,264,337,593,543,950,336 (2^96)
/31 131,072 33,554,432 8,589,934,592 158,456,325,028,528,675,187,087,900,672
/30 262,144 67,108,864 17,179,869,184 316,912,650,057,057,350,374,175,801,344
/29 524,288 134,217,728 34,359,738,368 633,825,300,114,114,700,748,351,602,688
/28 1,048,576 268,435,456 68,719,476,736 1,267,650,600,228,229,401,496,703,205,376
/27 2,097,152 536,870,912 137,438,953,472 2,535,301,200,456,458,802,993,406,410,752
/26 4,194,304 1,073,741,824 274,877,906,944 5,070,602,400,912,917,605,986,812,821,504
/25 8,388,608 2,147,483,648 549,755,813,888 10,141,204,801,825,835,211,973,625,643,008
/24 16,777,216 4,294,967,296 1,099,511,627,776 20,282,409,603,651,670,423,947,251,286,016
/23 33,554,432 8,589,934,592 2,199,023,255,552 40,564,819,207,303,340,847,894,502,572,032
/22 67,108,864 17,179,869,184 4,398,046,511,104 81,129,638,414,606,681,695,789,005,144,064
/21 134,217,728 34,359,738,368 8,796,093,022,208 162,259,276,829,213,363,391,578,010,288,128
/20 268,435,456 68,719,476,736 17,592,186,044,416 324,518,553,658,426,726,783,156,020,576,256
/19 536,870,912 137,438,953,472 35,184,372,088,832 649,037,107,316,853,453,566,312,041,152,512
/18 1,073,741,824 274,877,906,944 70,368,744,177,664 1,298,074,214,633,706,907,132,624,082,305,024
/17 2,147,483,648 549,755,813,888 140,737,488,355,328 2,596,148,429,267,413,814,265,248,164,610,048
/16 4,294,967,296 1,099,511,627,776 281,474,976,710,656 5,192,296,858,534,827,628,530,496,329,220,096
/15 8,589,934,592 2,199,023,255,552 562,949,953,421,312 1,038,459,371,706,965,525,706,099,265,844,0192
/14 17,179,869,184 4,398,046,511,104 1,125,899,906,842,624 207,691,874,341,393,105,141,219,853,168,80,384
/13 34,359,738,368 8,796,093,022,208 2,251,799,813,685,248 41,538,374,868,278,621,028,243,970,633,760,768
/12 68,719,476,736 17,592,186,044,416 4,503,599,627,370,496 83,076,749,736,557,242,056,487,941,267,521,536
/11 137,438,953,472 35,184,372,088,832 9,007,199,254,740,992 166,153,499,473,114,484,112,975,882,535,043,072
/10 274,877,906,944 70,368,744,177,664 18,014,398,509,481,984 332,306,998,946,228,968,225,951,765,070,086,144
/9 549,755,813,888 140,737,488,355,328 36,028,797,018,963,968 664,613,997,892,457,936,451,903,530,140,172,288
/8 1,099,511,627,776 281,474,976,710,656 72,057,594,037,927,936 1,329,227,995,784,915,872,903,807,060,280,344,576
/7 2,199,023,255,552 562,949,953,421,312 144,115,188,075,855,872 2,658,455,991,569,831,745,807,614,120,560,689,152
/6 4,398,046,511,104 1,125,899,906,842,624 288,230,376,151,711,744 5,316,911,983,139,663,491,615,228,241,121,378,304
/5 8,796,093,022,208 2,251,799,813,685,248 576,460,752,303,423,488 10,633,823,966,279,326,983,230,456,482,242,756,608
/4 17,592,186,044,416 4,503,599,627,370,496 1,152,921,504,606,846,976 21,267,647,932,558,653,966,460,912,964,485,513,216
/3 35,184,372,088,832 9,007,199,254,740,992 2,305,843,009,213,693,952 42,535,295,865,117,307,932,921,825,928,971,026,432
/2 70,368,744,177,664 18,014,398,509,481,984 4,611,686,018,427,387,904 85,070,591,730,234,615,865,843,651,857,942,052,864
/1 140,737,488,355,328 36,028,797,018,963,968 9,223,372,036,854,775,808 170,141,183,460,469,231,731,687,303,715,884,105,728
Aaron Hughes aaronh@bind.com http://www.bind.com