Class 20: Homework

1. The utility nslookup takes a fully qualified domain name for a host as a command-line argument and returns, among other things, the IP address of the host in dotted quad format. For example:

   bash$ nslookup www.nrl.navy.mil
   Server:         131.122.88.2
   Address:        131.122.88.2#53
   
   Non-authoritative answer:
   Name:   www.nrl.navy.mil
   Address: 132.250.119.6 ← IP Address for www.nrl.navy.mil
   	

   • Use nslookup to find (and write down!) the IP address of www.xkcd.com as a dotted quad:

   • Showing your work, convert this IP address from the dotted quad representation to a single number:

2. Suppose you have the following four bytes of memory representing a number:

   

   • What is the number assuming a big-endian representation? Show your work!

   • What is the number assuming a little-endian representation? Show your work?

   • What is the number in network byte order? Don't show any work!

3. The function inet_ntoa has the following prototype:

   char *inet_ntoa(const struct in_addr in);
   	

   where struct in_addr includes the following member:

   in_addr_t s_addr;
   	

   So, given the following definitions and prototypes:

   uint32_t htonl(uint32_t hostlong);
   uint32_t ntohl(uint32_t netlong);
   
   uint32_t A;
   struct in_addr B;
   char C;
   char *D;
   in_addr_t *E;
   	

   Give the type of each of the following expressions (write invalid for invalid expressions):

   a. &A
   b. *D
   c. htonl(A)
   d. B.s_addr
   e. &B.s_addr
   f. E->s_addr	  
   g. *E
   	

   ... and circle the valid statement calling inet_ntoa:

   C = inet_ntoa(B);       C = inet_ntoa(&B);
   
   
   D = inet_ntoa(B);       D = inet_ntoa(&B);
   
   

4. Explain why

   in_addr_t a = inet_addr(argv[1]);
   printf("%u\n",a);
   	

   correctly prints out an IP address for the dotted quad given in argv[1] on some machines, but not on others.