Information Technology Reference
In-Depth Information
the IP address sends an ARP reply message containing its MAC address. When a computer
learns another computer's MAC address, it keeps the address in its ARP cache temporarily so
that it doesn't have to send another ARP request packet to communicate with that computer
again. Entries in the ARP cache are kept for only a few minutes to prevent them from becom-
ing obsolete. Some options for the ARP command are as follows:
• -a, -g—Displays the contents of the ARP cache. These options perform the same function.
• -d—Deletes the entire contents of the ARP cache or a single entry specified by IP address.
This option can be useful if a computer's NIC has changed recently, and the cache contains
obsolete information.
• -s—Adds a permanent entry to the ARP cache by specifying a host's IP address and MAC
address. This option should be used only if the address of a frequently accessed computer
is unlikely to change. Remember: If the NIC is changed on a computer, its MAC address
changes as well.
The Tracert Command
Tracert is usually called “trace route” because it displays the route
packets take between two computers. Tracert displays the address or DNS name of each router a
packet travels through to reach the specified destination. It then sends a series of three ICMP Echo
Request packets with a TTL value starting at 1 and increases the value until the destination is
reached. Each router a packet encounters along the way to the destination decrements the TTL
value by 1. If the TTL value reaches 0, the router sends a TTL-expired message back to the send-
ing computer and drops the packet. When Tracert receives the TTL-expired message, it records
the sending router's IP address and the time to receive a reply and displays that information. Next,
a new series of three ICMP Echo Request packets are sent with an incremented TTL value. This
procedure continues until all routers between the source and destination have been recorded.
Tracert is useful for troubleshooting the routing topology of a complex network and finding
the bottleneck between a computer and a destination network. Because Tracert displays the time
it took to receive a reply from each router, a router (or the link to this router) showing an inor-
dinately long delay might be where the bottleneck lies.
The Nslookup Command
Nslookup is used to test and troubleshoot DNS operation
and can be used in command mode or interactive mode. In command mode, you type “nslookup
host
”;
host
is the name of a computer in the local domain or a fully qualified domain name.
Nslookup replies with the specified host's IP address. By default, Nslookup uses the DNS server
address configured in the IP address settings. Following are some examples of using Nslookup
in command mode:
nslookup server99
nslookup
www.yahoo.com
nslookup
www.google.com
172.31.1.200
The first two commands query the default DNS server. The last command queries a DNS
server at address 172.31.1.200. Because you can specify a different DNS server, you can com-
pare the results of different DNS servers to verify correct DNS operation.
To use interactive mode, just type “nslookup” at the command prompt, and nslookup dis-
plays which server it's using to perform lookups. You can type a question mark at the interac-
tive mode prompt to get a list of available options and commands.
Activity 8-3: Configuring a Second IP Address
Time Required:
15 minutes
Objective:
Add a second IP address to your server.
Description:
You want to test configuring multiple IP addresses on a server.
1. Log on to your server as Administrator, if necessary.
2. Click the
Network Connections
icon on the Quick Launch toolbar.
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