Networking I
Fall 1997
Questions:  IP Routing


1.  What unix command can be used to display the kernel routing table?

netstat -rn

2.  What are the possible flags which can be assigned to a route in the
    kernel routing table?  What does each mean?

	U - route is up
	H - Target is a host
	G - use gateway
	R - use dynamic routing
	D - dynamically installed
	M - modified 
	! - reject route


3.  Explain the concept of a default route.

Default route is a "next-hop" router that the datagram is forwarded to
if no other route can be found.

4.  Under unix, how is a network interface initialized (i.e. by what
    command)?

ifconfig

5.  If you are the superuser (root) on a unix machine which has an empty
    kernel routing table, how would you add a default route?

route add default gw baudwidth eth0

6.  What happens if an IP datagram is sent from a unix machine to a host for
    which the kernel does not have a direct route or a default route?  What
    message is returned to the application which sent the datatgram?

A "host unreachable" or "network
unreachable" error is normally returned to the application.

7.  (3.1) Must the loopback address be 127.0.0.1?

No, it can be any 127.x.x.x address.

8.  (3.2) Identify the routers in Figure 3.6 with more than two network
    interfaces.

kpno has five interfaces:  three point-to-point links and two ethernets.
R10 has four ethernet interfaces.  gateway has three interfaces: two
point-to-point links and one ethernet.  Finally, netb has one ehternet
interface and two point-to-point links.

9.  (3.3) What's the difference in the subnet mask for a class A address
    with 16 bits for the subnet ID and a class B address with 8 bits for the
    subnet ID?

There is no difference:  both have a subnet mask of 255.255.255.0, as does a
class C address that is not subnetted.

10. (3.5) Is the subnet mask 255.255.0.255 valid for a class A address?  See
    RFC 1219 [Tsuchiya 1991] for more info.  This is available from the 
    Networking I web page, as well as from the usual sources.

It's valid and it's called a noncontiguous subnet mask since the 16 bits for
the subnet mask are not contiguous.  The RFC's, however, recommend against
using noncontiguous subnet masks.