Bash + Core Utils, 43 Bytes seq $1|tac|factor|awk 'NF==2&&/666/&&$0=$2' Takes command line argument. Can be quite slow with larger numbers.

Bash + Core Utils, 51 49 Bytes seq $1|tac|factor|awk 'NF==2&&/666/&&!a--&&$0=$2' Takes command line argument. Can be quite slow with larger numbers.

Bash + bsd-games package, 35 primes 2 $[$1+1]|grep 666|tac|sed q Try it online.

Bash 37 31 27 bytes This is inspired by the answer by Bruce Forte. That answer only works with IPv4 which means it doesn't work for me. I have rewritten it into a version that (only) works with IPv6: ip r g b::|awk '$0=$9" "$9' This is tested on Ubuntu 14.04.5 connected through PPPoE and sinc...