is there another way to group the two blocks(x.x.128.0/21 and x.x.136.0/23) together so that they are both NATed to the same block(y.y.105.214 - y.y.105.254) with out using the mangle rule?
sorry, i haven't seen this. i think its a good way to handle blocks of ip-addresses.
i will do some NAT tests in the next weeks, as we have a few noncoherent public class-c ip-blocks we'd like to map into internal ip-ranges.
btw, what's about payload using NAT with mikrotik router os? will e.g. dns requests be handled similar to cisco NAT? i.e. translating dns-data in request but not in zone transfers. this is quite important for using public DNS-servers behind NAT and one reason we still use some cisco routers.