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The problem is that in the common case case those IP addresses are useless and serve no purpose. Broadcasts get broadcast because they are sent to broadcast MAC

The problem is that in the common case case those IP addresses are useless and serve no purpose. Broadcasts get broadcast because they are sent to broadcast MAC address. They can have IP packet with all-ones destination inside (which is different from a unicast packet sent to that same address via its unique MAC), they can have IP packet with a specific destination inside (well, maybe your ARP cache is malfunctioning, and you've forgotten the proper MAC address, but not the fact the host exists?), they can have arbitrary data (because you've invented our own protocol). You also need to call a different function or set some flag to make broadcast happen. It is possible that early IP stacks relied on a dedicated high-level broadcast address to prevent the need to handle all kinds of underlying networks in programs, but I'm not sure. (Trick question for the exam is whether broadcast traffic received by new host with assigned address of .255 would be a problem.) HOWEVER, if you want to broadcast something at a remote network, you need to have dedicated network broadcast IP addresses. Unfortunately, even RFC 919 from 1984 thinks it is not a recommended mode of operation. But imagine that we could get "ip/subnet" from DNS, send broadcast to a distant server farm, and receive response from anything that responded. Instead of DNS-based service discovery, we would have a single A record, and finally put that IANA port number database to use in port-based service discovery.

w7 3y ago
You can get some of properties you describe with the behavior most people would want via a combination of anycast, some route exchange protocol (probably BGP), and ECMP.
PinguTS 3y ago
It seems you assume, that IP runs on Ethernet. But what IP that runs on a different data link layer that uses different address and doesn't know what a MAC address is?
- ogurechny 3y ago
  Then it uses a different broadcast method that also does not depend on higher level IP addresses. My point is that high level abstractions, like dedicated broadcast IP address that magically alters the behavior of lower level components, complicate things. Say, there is SO_BROADCAST, but it does nothing for TCP (even though we can imagine how broadcast client-to-server connection initiation might work), and there seems to be disagreement whether it allows to use broadcast address, or instructs to use broadcasts with any address. But Berkley sockets are also from mid-'80s, and it is possible that previous alternatives required manual configuration of low level APIs on each individual supported network to use broadcasts, so that was a step forward.

w7 3y ago

You can get some of properties you describe with the behavior most people would want via a combination of anycast, some route exchange protocol (probably BGP), and ECMP.

PinguTS 3y ago

It seems you assume, that IP runs on Ethernet. But what IP that runs on a different data link layer that uses different address and doesn't know what a MAC address is?

ogurechny 3y ago
Then it uses a different broadcast method that also does not depend on higher level IP addresses. My point is that high level abstractions, like dedicated broadcast IP address that magically alters the behavior of lower level components, complicate things. Say, there is SO_BROADCAST, but it does nothing for TCP (even though we can imagine how broadcast client-to-server connection initiation might work), and there seems to be disagreement whether it allows to use broadcast address, or instructs to use broadcasts with any address. But Berkley sockets are also from mid-'80s, and it is possible that previous alternatives required manual configuration of low level APIs on each individual supported network to use broadcasts, so that was a step forward.