#!/bin/sh
# the next line restarts using tclsh \
exec tclsh "$0" "$@"

proc getIp {s} {
    set l [split $s .]
    set n1 [expr {[lindex $l 0] << 24}]
    set n2 [expr {[lindex $l 1] << 16}]
    set n3 [expr {[lindex $l 2] << 8}]
    set n4 [lindex $l 3]
    return [expr {$n1 | $n2 | $n3 | $n4}]
}

proc getDottedIp {ip} {
    set n1 [expr {($ip >> 24) & 0xff}]
    set n2 [expr {($ip >> 16) & 0xff}]
    set n3 [expr {($ip >> 8) & 0xff}]
    set n4 [expr {$ip & 0xff}]
    return "$n1.$n2.$n3.$n4"
}

if {$argc < 2} {
    puts "usage: getsubnet <host> <mask>"
    exit 0
}

if {! [regexp {^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$} [lindex $argv 0]]} {
    puts "error: invalid host format"
    exit 1
}

if {! [regexp {^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$} [lindex $argv 1]]} {
    puts "error: invalid mask format"
    exit 1
}

set ip [getIp [lindex $argv 0]]
set mask [getIp [lindex $argv 1]]

puts [getDottedIp [expr {$ip & $mask}]]
.