Information Technology Reference
In-Depth Information
These and other network applications use the services of TCP/IP and other lower-layer Internet protocols
to provide users with basic network services.
Domain Name System
TCP/IP uses a numeric addressing scheme in which each node is assigned an IP address that is used to
route packets to a node on the network. Because it is much easier for people to remember names such as
www.somedomain.com instead of 10.1.1.1, a protocol called Domain Name System (DNS) is used to
map numbers to names, and vice versa. Most web pages refer to other web pages or links using these
names instead of their IP addresses. This provides many advantages; for example, the address can change
without breaking any links to a web page if the DNS table is also changed to point to the new address.
Tools for Troubleshooting IP Problems
The tools ping and traceroute, both in the TCP/IP protocol suite, will greatly assist in troubleshooting
IP connectivity. Most operating systems and IP implementations come with these tools installed by
default. On some UNIX platforms, however, you may need to download and install a traceroute package.
Cisco routers provide a basic method of viewing IP traffic switched through the router called
packet
debugging
. Packet debugging enables a user to determine whether traffic is travelling along an expected
path in the network or whether there are errors in a particular TCP stream. Although in some cases packet
debugging can eliminate the need for a packet analyzer, it should not be considered a replacement for
this important tool.
Packet debugging can be very intrusive—in some cases, it can cause a router to become inoperable until
physically reset. In other instances, packets that are present on the network and switched through the
router may not be reported by packet debugging. Thus, a firm conclusion cannot be drawn that a packet
was not sent solely from the output of packet debugging; a network analyzer must be used to accurately
make this assessment.
Packet debugging should be used with extreme caution by only advanced
operators because it can cause the router to lock up and stop routing traffic, if not used carefully.
The
risks of using packet debugging may be compounded by the necessity of disabling fast switching for
packet debugging to be effective. As a general rule, packet debugging should not be used on a production
router unless you have physical access to the router and are willing to risk it going down.
ping
The ping tool uses the IP ICMP echo request and echo reply messages to test reachability to a remote
system. In its simplest form, ping simply confirms that an IP packet is capable of getting to and getting
back from a destination IP address (Figure 7-7). This tool generally returns two pieces of information:
whether the source can reach the destination (and, by inference, vice versa), and the round-trip time
(RTT, typically in milliseconds). The RTT returned by ping should be used only as a comparative
reference because it can depend greatly on the software implementation and hardware of the system on
which ping is run. If ping fails or returns an unusual RTT, traceroute can be used to help narrow down
the problem. It is also possible to vary the size of the ICMP echo payload to test problems related to
maximum transmission unit (MTU).
