15.6

Faster systems may be able to send more packets in a shorter amount of time. The network would then have more packets traveling on it, resulting in more collisions, and therefore less throughput relative to the number of packets being sent. More networks can be used with fewer systems per network to reduce the number of collisions.

15.8

Why would it be a bad idea for gateways to pass broadcast packets between networks?

performace: If gateways passed all broadcast packets between networks then there would be a congestion problem so bad that the Internet would probably be useless.
security: It may be a security problem for some packets to be sent outside a single administrative domain.

What would be the advantages of doing so?

Broadcast and multicast are useful for regularly changing data that is very popular, or of global interest. Multicasting changes to the home page for www.cnn.com, for example, might be a good idea.
Broadcast communication has the advantage that you don't need to know the location of the thing you are communicating with. This means you don't need name servers (to map names to network addresses) or routing tables (to map network addresses to directions).

15.9

In what ways is using a name server better than using static host tables?

Much less storage required for clients because you only need to store the name<->address mappings you're using, rather than all 200 zillion mappings.
It's much easier to add or change mappings because the mappings are stored on one server and fetched as needed from there. With static tables, each host needs to download a 1GB (or whatever) file every night to get updates.
DNS is nice because each administrative domain has total control over its names. If administrators at the UW CSE dept. need to add a machine "foo", they can just call it "foo.cs.washington.edu", without having to talk to anyone else in the world to get approval.
Name servers allow flexibility in scaling services to handle more requests per second by mapping a single name to multiple IP addresses dynamically. Yahoo and CNN use DNS "round robin" for example, to map "www.yahoo.com" and "www.cnn.com" to multiple machines.

What are the problems and complications associated with name servers?

If the name server is down and you don't know the IP address of the machine you're trying to reach and you don't have a name<->address mapping cached, it's impossible to reach the host.
Message-passing over the network to fetch the mapping from the name server is much slower (~100ms) than fetching it from disk (~5-10ms).
Making scalable name servers that can handle enough requests per second may be a problem for the root name servers or heavily loaded web sites.

What methods could be used to decrease the amount of traffic name servers generate to satisfy translation requests?

One word: Caching. The only way to avoid sending a request is if you already have the answer.