[Old Pedant]

By the by, you could get rid of those useless id's in the fields, you know.

function moveMax(pfield,nfield)
{
  if(pfield.value.length >= pfield.maxLength)
   {
       pfield.form[nfield].focus();
   }
}

//inputs
<input type="text" name="domainIPa" value="" size="2" maxlength="3" onkeyup="moveMax(this,'domainIPb');" /> -
<input type="text" name="domainIPb" value="" size="2" maxlength="3" onkeyup="moveMax(this,'domainIPc');" /> -
<input type="text" name="domainIPc" value="" size="2" maxlength="3" onkeyup="moveMax(this,'domainIPd');" /> -
<input type="text" name="domainIPd" value="" size="2" maxlength="3" onkeyup="moveMax(this,'domainIPa');" />