Output IP addresses for machines connected to the local machine using /proc/net/tcp for speed

connected_ips.awk

#!/usr/bin/awk -f
# Output IP addresses for machines that are connected to the local machine.
#
# Usage:
# ./connected_ips.awk /proc/net/tcp
# ./connected_ips.awk -v port=4505 /proc/net/tcp

function fromhex(s){
    # Amazing voodoo from William.
    # http://compgroups.net/comp.lang.awk/reading-hexadecimal-numbers/33952
    return index("0123456789abcdef", tolower(substr(s, length(s)))) \
        -1 + (sub(/.$/, "" ,s) ? 16 * fromhex(s) : 0)
}

BEGIN {
    port_match = port ? sprintf(":%X$", port) : ":*$"
}

# Skip the header line and loopback addresses; optionally filter by port.
NR != 1 && $3 !~ /:0000$/ && $2 ~ port_match {
    split($3, conn, ":"); ip = conn[1]

    if (!port) { sub(/[A-F0-9]+:/, "", $2); printf("%d\t", fromhex($2)) }

    printf("%d.%d.%d.%d\n", \
        fromhex(substr(ip, 7, 2)),
        fromhex(substr(ip, 5, 2)),
        fromhex(substr(ip, 3, 2)),
        fromhex(substr(ip, 0, 2)))
}