IPv4 – Directed and Limited Broadcast Confusion Explained

What happens is desirable. It would not be desirable for an interface with multiple IP addresses to have the ARP cache changed every time one of the different addresses is used. The ARP cache will naturally timeout MAC addresses. You can probably clear the ARP cache, and it will get rebuilt naturally.

The main benefit of hashing rather than a simple compare is that you can do one lookup for any number of enabled multicast addresses. The system that has an exact match for 80 addresses will either have to build logic to fetch 80 addresses into a comparator in turn, or have 80 comparators in parallel. That's a lot of gates, even if it is simple in concept. And that's all extra cost and power.

In contrast, a hash lookup can be easily implemented with a shift register and a few XOR gates. The computation and shifts can be done as the bits arrive on the wire. Even better, the NIC is doing this already, in order to calculate the checksum.

Note also, that Stevens was writing 20-25 years ago; whilst it is still more or less the definitive work on TCP/IP, hardware has come along since then. The cost of adding a few thousand more gates won't make so much difference. Flicking through a few NIC datasheets, most opt for a hybrid approach: e.g. 16 exact addresses and a 4096 bit hashtable.