reverse a string or list

or a slightly more generic approach in case you don´t have a fixed number of digits and want to use list functions:

set ip_addr [IP::client_addr]
set separator "."
for { set digit [expr {[llength  [split $ip_addr $separator]] -1 }] } { $digit >= 0 } { incr digit -1 } {
    lappend reverse_order [lindex [split $ip_addr $separator] $digit]
}
set reverse_ip [join $reverse_order $separator]

But for sure Brad´s solution is way faster. +1 🙂