# phpman > man > cidr_table(5) [CIDR_TABLE(5)](https://www.chedong.com/phpMan.php/man/CIDRTABLE/5/markdown) File Formats Manual [CIDR_TABLE(5)](https://www.chedong.com/phpMan.php/man/CIDRTABLE/5/markdown) ## NAME cidr_table - format of Postfix CIDR tables ## SYNOPSIS **postmap** **-q** **"**_string_**"** **cidr:/etc/postfix/**_filename_ **postmap** **-q** **-** **cidr:/etc/postfix/**_filename_ **<**_inputfile_ ## DESCRIPTION The Postfix mail system uses optional lookup tables. These tables are usually in **dbm** or **db** format. Alternatively, lookup tables can be specified in CIDR (Classless Inter-Domain Rout‐ ing) form. In this case, each input is compared against a list of patterns. When a match is found, the corresponding result is returned and the search is terminated. To find out what types of lookup tables your Postfix system supports use the "**postconf** **-m**" command. To test lookup tables, use the "**postmap** **-q**" command as described in the SYNOPSIS above. ## TABLE FORMAT The general form of a Postfix CIDR table is: _pattern_ _result_ When a search string matches the specified _pattern_, use the corresponding _result_ value. The _pattern_ must be in _network/prefix_ or _network_address_ form (see ADDRESS PAT‐ TERN SYNTAX below). **!**_pattern_ _result_ When a search string does not match the specified _pattern_, use the specified _result_ value. The _pattern_ must be in _network/prefix_ or _network_address_ form (see ADDRESS PAT‐ TERN SYNTAX below). This feature is available in Postfix 3.2 and later. **if** _pattern_ **endif** When a search string matches the specified _pattern_, match that search string against the patterns between **if** and **endif**. The _pattern_ must be in _network/prefix_ or _net__‐ _work_address_ form (see ADDRESS PATTERN SYNTAX below). The **if**..**endif** can nest. Note: do not prepend whitespace to text between **if**..**endif**. This feature is available in Postfix 3.2 and later. **if** **!**_pattern_ **endif** When a search string does not match the specified _pattern_, match that search string against the patterns between **if** and **endif**. The _pattern_ must be in _network/prefix_ or _network_address_ form (see ADDRESS PATTERN SYNTAX below). The **if**..**endif** can nest. Note: do not prepend whitespace to text between **if**..**endif**. This feature is available in Postfix 3.2 and later. blank lines and comments Empty lines and whitespace-only lines are ignored, as are lines whose first non-white‐ space character is a `#'. multi-line text A logical line starts with non-whitespace text. A line that starts with whitespace continues a logical line. ## TABLE SEARCH ORDER Patterns are applied in the order as specified in the table, until a pattern is found that matches the search string. ## ADDRESS PATTERN SYNTAX Postfix CIDR tables are pattern-based. A pattern is either a _network_address_ which requires an exact match, or a _network_address/prefix_length_ where the _prefix_length_ part specifies the length of the _network_address_ prefix that must be matched (the other bits in the _network_ad__‐ _dress_ part must be zero). An IPv4 network address is a sequence of four decimal octets separated by ".", and an IPv6 network address is a sequence of three to eight hexadecimal octet pairs separated by ":" or "::", where the latter is short-hand for a sequence of one or more all-zero octet pairs. The pattern 0.0.0.0/0 matches every IPv4 address, and ::/0 matches every IPv6 address. IPv6 sup‐ port is available in Postfix 2.2 and later. Before comparisons are made, lookup keys and table entries are converted from string to bi‐ nary. Therefore, IPv6 patterns will be matched regardless of leading zeros (a leading zero in an IPv4 address octet indicates octal notation). Note: address information may be enclosed inside "[]" but this form is not required. ## EXAMPLE SMTPD ACCESS MAP /etc/postfix/main.cf: smtpd_client_restrictions = ... cidr:/etc/postfix/client.cidr ... /etc/postfix/client.cidr: # Rule order matters. Put more specific allowlist entries # before more general denylist entries. 192.168.1.1 OK 192.168.0.0/16 REJECT 2001:[db8::1](https://www.chedong.com/phpMan.php/perldoc/db8%3A%3A1/markdown) OK 2001:db8::/32 REJECT ## SEE ALSO [postmap(1)](https://www.chedong.com/phpMan.php/man/postmap/1/markdown), Postfix lookup table manager [regexp_table(5)](https://www.chedong.com/phpMan.php/man/regexptable/5/markdown), format of regular expression tables [pcre_table(5)](https://www.chedong.com/phpMan.php/man/pcretable/5/markdown), format of PCRE tables ## README FILES Use "**postconf** **readme**___**directory**" or "**postconf** **html**___**directory**" to locate this information. DATABASE_README, Postfix lookup table overview ## HISTORY CIDR table support was introduced with Postfix version 2.1. **AUTHOR(S)** The CIDR table lookup code was originally written by: Jozsef Kadlecsik KFKI Research Institute for Particle and Nuclear Physics POB. 49 1525 Budapest, Hungary Adopted and adapted by: Wietse Venema IBM T.J. Watson Research P.O. Box 704 Yorktown Heights, NY 10598, USA Wietse Venema Google, Inc. 111 8th Avenue New York, NY 10011, USA [CIDR_TABLE(5)](https://www.chedong.com/phpMan.php/man/CIDRTABLE/5/markdown)