Sorting IPv6

Friday, May 23, 2025

Recently, Chris Siebenmann was lamenting the lack of a good command line way to sort IPv6 addresses. This followed a post of his a few years ago about how sort -V can easily sort IPv4 addresses. Since I had some fun talking about sorting Roman numerals recently – and we have an extensive standard library – I thought I’d talk about how you might solve this problem with Factor.

As a reminder, IPv6 uses a 128-bit address space with more theoretical addresses than the older – but still quite commonly used – IPv4 32-bit address space.

The internal network address of your computer is sometimes referred to as localhost or a loopback address, and represented as 127.0.0.1 in IPv4, or ::1 in IPv6. We have an ip-parser vocabulary with words for parsing and manipulating IP addresses as well as IP network strings written in CIDR notation. We can use these words to show how to translate these addresses to their byte representation:

IN: scratchpad "127.0.0.1" parse-ipv4 .
B{ 127 0 0 1 }

IN: scratchpad "::1" parse-ipv6 .
{ 0 0 0 0 0 0 0 1 }

And, we could use that to sort a list of addresses pretty easily:

IN: scratchpad {
                   "127.0.0.1"
                   "1.1.1.1"
                   "8.8.8.8"
                   "192.168.10.40"
               } [ parse-ipv4 ] sort-by .
{
    "1.1.1.1"
    "8.8.8.8"
    "127.0.0.1"
    "192.168.10.40"
}

IN: scratchpad {
                   "2620:0:1cfe:face:b00c::3"
                   "2001:4860:4860::8844"
                   "2620:0:ccc::2"
                   "::1"
                   "2001:4860:4860::8888"
               } [ parse-ipv6 ] sort-by .
{
    "::1"
    "2001:4860:4860::8844"
    "2001:4860:4860::8888"
    "2620:0:ccc::2"
    "2620:0:1cfe:face:b00c::3"
}

And so, now that some great feedback encouraged us to do command-line eval with auto-use? enabled, we can run this easily as a one-line script:

# make a file full of unsorted IPv6 addresses
$ cat <<EOF > ips.txt
2620:0:1cfe:face:b00c::3
2001:4860:4860::8844
2620:0:ccc::2
::1
2001:4860:4860::8888
EOF

# show that you can parse the file as strings
$ cat ips.txt | ./factor -e="read-lines ."
{
    "2620:0:1cfe:face:b00c::3"
    "2001:4860:4860::8844"
    "2620:0:ccc::2"
    "::1"
    "2001:4860:4860::8888"
}

# sort and print the sorted output
$ cat ips.txt | ./factor -e="read-lines [ parse-ipv6 ] sort-by [ print ] each"
::1
2001:4860:4860::8844
2001:4860:4860::8888
2620:0:ccc::2
2620:0:1cfe:face:b00c::3

Pretty cool!