Code:

proc ipv6tonibble {ip {type "int"}} {  set newip ""  set nlist ""  set mlist [split $ip ":"]  foreach element $mlist {    if {[string trim $element] == ""} {      set totzeros [expr 8 - [llength $mlist]]      for {set i 0} {$i <= $totzeros} {incr i} {        lappend nlist 0      }    } else {      lappend nlist $element    }  }  for {set i [expr [llength $nlist] - 1]} {$i >= 0} {incr i -1} {    set seg [lindex $nlist $i]    for {set j 0} {$j < 4} {incr j} {      if {$seg != ""} {        append newip "[string index $seg [expr [string length $seg] - 1]]."        if {[string length $seg] > 1} {          set seg [string range $seg 0 [expr [string length $seg] - 2]]        } else {          set seg ""        }      } else {        append newip "0."      }    }  }  append newip "ip6.$type"  return $newip}