(i) DNS
(ii) Time to live
(iii) Header checksum
(iv) Fragment offset
(v) Type of service
Answer:
(i) DNS:
Programs rarely refer to hosts, mailboxes and other resources by their binary network addresses. Way back in the ARPANET, there are simply a file, hosts.txt that listed all the hosts and their IP addresses. However, when thousands of workstations were connected to the net, everyone realize that this approach could not continue to work forever. For one thing, the size of the file would become too large. However, even more important, host name conflicts would occur constantly unless names were centrally managed, something unthinkable in a huge international network. To solve these problems, DNS (the Domain Name System) was invented.
To map a name onto an IP address, an application program calls a library procedure called the resolver, passing it the name as a parameter. The resolver sends a UDP packet to a local DNS server, which then looks up the name and returns the IP address to the resolver, which then returns it to the caller. Armed with the IP address, the program can then establish a TCP connection with the destination, or send it UDP packets.
Three characteristics of DNS
(1) Hierarchical naming scheme
(2) distributed databases of name to IP address mapping
(3) Delegation of authority for names.