Max length for client ip address

Take it from someone who has tried it all three ways... just use a varchar(39)

The slightly less efficient storage far outweighs any benefit of having to convert it on insert/update and format it when showing it anywhere.

For IPv4, you could get away with storing the 4 raw bytes of the IP address (each of the numbers between the periods in an IP address are 0-255, i.e., one byte). But then you would have to translate going in and out of the DB and that's messy.

IPv6 addresses are 128 bits (as opposed to 32 bits of IPv4 addresses). They are usually written as 8 groups of 4 hex digits separated by colons: 2001:0db8:85a3:0000:0000:8a2e:0370:7334. 39 characters is appropriate to store IPv6 addresses in this format.

Edit: However, there is a caveat, see @Deepak's answer for details about IPv4-mapped IPv6 addresses. (The correct maximum IPv6 string length is 45 characters.)

If you want to handle IPV6 in standard notation there are 8 groups of 4 hex digits:

2001:0dc5:72a3:0000:0000:802e:3370:73E4

32 hex digits + 7 separators = 39 characters.

CAUTION: If you also want to hold IPV4 addresses mapped as IPV6 addresses, use 45 characters as @Deepak suggests.

There's a caveat with the general 39 character IPv6 structure. For IPv4 mapped IPv6 addresses, the string can be longer (than 39 characters). An example to show this:

IPv6 (39 characters) :

ABCD:ABCD:ABCD:ABCD:ABCD:ABCD:ABCD:ABCD

IPv4-mapped IPv6 (45 characters) :

ABCD:ABCD:ABCD:ABCD:ABCD:ABCD:192.168.158.190

Note: the last 32-bits (that correspond to IPv4 address) can need up to 15 characters (as IPv4 uses 4 groups of 1 byte and is formatted as 4 decimal numbers in the range 0-255 separated by dots (the . character), so the maximum is DDD.DDD.DDD.DDD).

The correct maximum IPv6 string length, therefore, is 45.

This was actually a quiz question in an IPv6 training I attended. (We all answered 39!)