Help:Range blocks

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See Help:Range blocks/IPv6 for information on IPv6 range blocks.

Range blocks are technical restrictions applied through Special:Block to a group of IP addresses that prevent them from editing, creating new accounts, sending email through the wiki interface, etc. If you check the box "Apply block to logged-in users from this IP address", edits from registered accounts will also be disabled while they are connecting from the blocked range.

To block an IP range from Special:Block, enter the first IP address in the range followed by a forward slash and a Classless inter-domain routing (CIDR) suffix. You should avoid performing range blocks unless you understand what you are doing, or you may end up blocking tens of thousands or even millions of people who are not the problem!

This article mainly discusses IPv4; IPv6 blocks work similarly, but have different implications—see /IPv6.

Non-technical explanation

IP addresses are broken up into blocks of numbers. An example of this would be 148.20.57.0 through to 148.20.57.255. Once it reaches 255 the next number is 148.20.58.0.

IP addresses can be broken up in to smaller or larger blocks. The smallest practical block is a block of 4. This could be one of the following:

148.20.57.0 - 148.20.57.3,

148.20.57.4 - 148.20.57.7,

148.20.57.8 - 148.20.57.11, ...

Of each block of 4 numbers, only two can be assigned to a computer. The first and last numbers of any block are reserved for network communication. These are level 30 blocks and can be expressed like this:

148.20.57.0/30,

148.20.57.4/30,

148.20.57.8/30, ...

The next largest block is 8. They can be as follows:

148.20.57.0 - 148.20.57.7,

148.20.57.8 - 148.20.57.15,

148.20.57.16 - 148.20.57.23, ...

In this block of 8 numbers only 6 can be assigned to a computer as, once again, the first and last numbers in a block are reserved for specific uses in network communication. These can also be expressed as follows:

148.20.57.0/29,

148.20.57.8/29,

148.20.57.16/29, ...

From this point on, the number of IP addresses in a block continues to double: 16, 32, 64, 128, 256, etc.

A block of 16 would start 148.20.57.0/28.

A block of 32 would start 148.20.57.0/27.

A block of 64 would start 148.20.57.0/26.

A block of 128 would start 148.20.57.0/25.

A block of 256 would start 148.20.57.0/24.

So if you have an IP address and you want to block the range assigned how do you know which one to use? Let's say you have a problem with 148.20.57.34. You can look up who has this IP address at http://arin.net/whois/?queryinput=148.20.57.34. Say this tells us that this IP address is assigned, along with a LOT of others in a /17 range, to the Department of Defense. We certainly don't want to block a large block of the DoD! The rule of thumb is block as little as possible. Only block a range if there is a cluster of IP addresses giving a problem.

There's a calculator that is very useful for this:

toolforge:ftools/general/ip-range-calc.html

Go to this site and enter 148.20.57.34 into the first set of blanks. Now select Network Prefix Length and enter 27 (this will give a block of 32 addresses) and click Calculate Network Information. This will show us a block of 32 IP addresses that include 148.20.57.34. (The first—network and the last—broadcast addresses will be displayed along with the usable addresses in the range.) You can use this tool to test ranges to be sure they are what you want before entering the information to initiate the block.

Technical explanation

CIDR notation is written as the IP address, a slash, and the CIDR suffix (for example, the IPv4 "10.2.3.41/24" or IPv6 "a3:bc00::/24"). The CIDR suffix is the number of starting digits every IP address in the range have in common when written in binary.

For example: "10.10.1.32" is binary "00001010.00001010.00000001.00100000", so 10.10.1.32/27 will match the first 27 digits ("00001010.00001010.00000001.00100000"). The IP addresses 10.10.1.32–10.10.1.63, when converted to binary, all have the same 27 first digits and will be blocked if 10.10.1.32/27 is blocked.

As the CIDR suffix increases, the block affects fewer IP addresses (see table of sample ranges). CIDR suffixes are not the same for IPv4 addresses as they are for IPv6 addresses; the same CIDR suffix in IPv4 blocks $2^{96}$ =79,228,162,514,264,337,593,543,950,336 times as many addresses in IPv6.

Calculating the CIDR suffix

You can use the table of sample ranges below to guess the range, use a computer script, or manually calculate the range.

Conversion to binary

The first step in manually calculating a range is to convert the first and last IP address to binary representation. (This assumes you're not using a computer script, which can probably calculate the range for you anyway.) An IP address is composed of four groups of eight ones and zeros. Each group represents a number from 0 to 255. To convert a number to binary, you can use a reference table or know the value of each binary digit:

Binary digit:	`1 1 1 1 1 1 1 1`
Value:	`128 64 32 16 8 4 2 1`

Proceeding from left to right, fill in 1 if the number is at least that value, and subtract that value (if it's not, fill in 0 and don't subtract). For example, to calculate 240:

240 is at least 128, so place 1 and subtract 128.
112 (240-128) is at least 64, so place 1 and subtract 64.
48 (112-64) is at least 32, so place 1 and subtract 32.
16 (48-32) is at least 16, so place 1 and subtract 16.
Since the remaining value is zero, all the remaining places are 0.

Thus, 240 is 1111 0000 because it can be represented as 128+64+32+16+0+0+0+0.

Calculate range

Place both IP addresses one atop the other, and count how many starting digits are exactly alike. This is the CIDR suffix.
Double-check! Being off by one digit could extend your block by thousands of addresses.

The example below calculates the CIDR range between 69.208.0.0 and 69.208.0.255. Note that this is a simple example; some groups of IP addresses do not so neatly fit CIDR suffixes, and need multiple different-sized blocks to block the exact range.

IP addresses:

69.208.0.0

69.208.0.255

Convert to binary:

0100 0101.1101 0000.0000 0000.0000 0000

0100 0101.1101 0000.0000 0000.1111 1111

Count identical first numbers:

0100 0101.1101 0000.0000 0000.0000 0000

0100 0101.1101 0000.0000 0000.1111 1111

|____________________________|

24 digits

CIDR range:

69.208.0.0/24

Table of sample ranges

The table below shows the IPv4 blocks each CIDR suffix affects. Note that MediaWiki only supports blocking CIDR suffixes 16 - 32 in IPv4 and 19 (formerly 64) - 128 in IPv6 by default (subject to $wgBlockCIDRLimit ). See /IPv6 for an IPv6 range table.

Example IP4 ranges
CIDR	Start Range	End Range	Total addresses	Bits selected in IP address
69.208.0.0/0	0.0.0.0	255.255.255.255	4,294,967,296	******.****.****.******
69.208.0.0/1	0.0.0.0	127.255.255.255	2,147,483,648	0*****.****.****.******
69.208.0.0/4	64.0.0.0	79.255.255.255	268,435,456	0100**.****.****.******
69.208.0.0/8	69.0.0.0	69.255.255.255	16,777,216	01000101.******.****.******
69.208.0.0/11	69.192.0.0	69.223.255.255	2,097,152	01000101.110***.****.******
69.208.0.0/12	69.208.0.0	69.223.255.255	1,048,576	01000101.1101**.****.******
69.208.0.0/13	69.208.0.0	69.215.255.255	524,288	01000101.11010*.****.******
69.208.0.0/14	69.208.0.0	69.211.255.255	262,144	01000101.110100.****.******
69.208.0.0/15	69.208.0.0	69.209.255.255	131,072	01000101.1101000.*****.******
69.208.0.0/16	69.208.0.0	69.208.255.255	65,536	01000101.11010000.******.******
69.208.0.0/17	69.208.0.0	69.208.127.255	32,768	01000101.11010000.0*****.******
69.208.0.0/18	69.208.0.0	69.208.63.255	16,384	01000101.11010000.00****.******
69.208.0.0/19	69.208.0.0	69.208.31.255	8,192	01000101.11010000.000***.******
69.208.0.0/20	69.208.0.0	69.208.15.255	4,096	01000101.11010000.0000**.******
69.208.0.0/21	69.208.0.0	69.208.7.255	2,048	01000101.11010000.00000*.******
69.208.0.0/22	69.208.0.0	69.208.3.255	1,024	01000101.11010000.000000.******
69.208.0.0/23	69.208.0.0	69.208.1.255	512	01000101.11010000.0000000.*******
69.208.0.0/24	69.208.0.0	69.208.0.255	256	01000101.11010000.00000000.********
69.208.0.0/25	69.208.0.0	69.208.0.127	128	01000101.11010000.00000000.0*******
69.208.0.0/26	69.208.0.0	69.208.0.63	64	01000101.11010000.00000000.00******
69.208.0.0/27	69.208.0.0	69.208.0.31	32	01000101.11010000.00000000.000*****
69.208.0.0/28	69.208.0.0	69.208.0.15	16	01000101.11010000.00000000.0000****
69.208.0.0/29	69.208.0.0	69.208.0.7	8	01000101.11010000.00000000.00000***
69.208.0.0/30	69.208.0.0	69.208.0.3	4	01000101.11010000.00000000.000000**
69.208.0.0/31	69.208.0.0	69.208.0.1	2	01000101.11010000.00000000.0000000*
69.208.0.0/32	69.208.0.0	69.208.0.0	1	01000101.11010000.00000000.00000000

Default limitation

The default MediaWiki installation limits range blocks to no larger than /16 IPv4 rangeblocks (65,536 addresses). To block larger ranges $wgBlockCIDRLimit needs to be set accordingly in LocalSettings.php .

Known problems

One important already-known problem caused by any range-block, is that as side-effect they also block some trusted registered groups, like wiki administrators, users who do not need to be patrolled by others, and trusted bots. Details: phabricator:T309328

References

Classless Inter-Domain Routing

External links

Netmask calculator which helps in making the correct decision for range blocks.
Subnet Calculator can help calculate prefix length and subnet mask for IPv4 and IPv6.
toolforge:ftools/general/ip-range-calc.html gives you the range you should use when blocking.
IPv4 and CIDR Calculator gives you a breakdown of Hosts and IP Range for any Given Mask/CIDR and reverse.