Serial Links and the /30 mask

When putting serial links on routers, there will only ever be TWO hosts - the serial ports at each end of the link.
Therefore it makes no sense to assign a whole /24 (Class C) network to a serial link.

1.  How many addresses would be wasted if a /24 network was used to provide addresses on a serial link?

The solution is to use a much smaller sized network. The convention is to use a /30 subnetwork (255.255.255.252).
A / 30 subnetwork is the smallest practical network that is possible in IP version 4.
This is known as Variable Length Subnet Masking (VLSM) and allows for Classless Interdomain Routing (CIDR).

VLSM and CIDR have been adopted since the early 1990s.

A / 30 subnetwork has FOUR addresses e.g.

192.168.100.0       Network or wire address
192.168.100.1       First serial interface address
192.168.100.2       Second serial interface address
192.168.100.3       Broadcast address

As there are only two host addresses on a serial link there will be no wastage of addresses.

The next FOUR numbers could be used on another serial link e.g.

192.168.100.4       Network or wire address
192.168.100.5       First serial interface address
192.168.100.6       Second serial interface address
192.168.100.7       Broadcast address

For all serial links from now on, you must use a /30 address. These are generally RFC 1918 addresses.

2. What is special about RFC 1918 addresses?

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Setting Interface Addresses

Note that for this lab, all commands are in courier font.

The IP addressing scheme is given in the diagram above.
You should use a /24 mask for the local area (Ethernet) network.

Make a copy of this diagram on the A3 paper provided for you.
Write the tables of IP addressing information on the diagram too (see below).

Using your mouse pointer, hover over each of the FastEthernet and Serial links in Packet Tracer to discover which router port it is connected to.
Make a table of connections for each router and enter the IP address and subnet mask for the connected interfaces similar to that below for all THREE routers.

3. What type of cable is used between the Internet and Chatham routers?

Note that each serial link has a small icon representing a clock at ONE end.
This end of the serial link, known as DCE, supplies the timing to the other end of the link.
Make a note which interfaces are responsible for supplying the clocking.
In this lab we will use 56000 bps as the clock rate for the serial links.

Chatham Interface Information
Interface	IP Address	Subnet Mask	Clock Rate	Default Gateway
Serial0/0				N/A
Serial0/1				N/A

Rochester Interface Information
Interface	IP Address	Subnet Mask	Clock Rate	Default Gateway
FastEthernet0/0			N/A
Serial0/0				N/A

Internet Interface Information
Interface	IP Address	Subnet Mask	Clock Rate	Default Gateway
Serial0/0				N/A

5. Make a copy of these tables available in your logbook.

6. Why is there no need for a Default Gateway on a serial network?

Once you have completed all THREE tables, you can begin entering the interface addresses in a similar way to that shown below:

Chatham>enable
Chatham#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Chatham(config)#interface serial 0/0
Chatham(config-if)#ip address 192.168.100.2 255.255.255.252
Chatham(config-if)#clock rate 56000
Chatham(config-if)#no shutdown
Chatham(config-if)#end
Chatham#write


Repeat this procedure to supply addresses and subnet mask for all connected interfaces on all THREE routers.

Refer to last week's lab if you are unsure how to enter the IP information.

Add the interface addresses for the other TWO routers using the IP addressing information that you wrote in the tables.

Remember to put a clock rate on all serial links that are the DCE end of the link.

When you have completed this, all links should have a green dot to show that the interfaces are 'up'.

If any of the links remain red, you should check you have entered the data correctly.

At the Priv Exec prompt enter the command show ip interface brief for each router

Chatham#show ip interface brief
Interface              IP-Address         OK? Method Status                Protocol
 
FastEthernet0/0        [your ip address]  YES manual up                    up
 
FastEthernet0/1        unassigned         YES manual administratively down down
 
Serial0/0              [your ip address]  YES manual down                  down
 

The Status and Protocol should both appear 'up' not 'down' as highlighted above. 'down' indicates a problem with the configuration.

7. Make a copy of the output of this command for each router available in your logbook.

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Remember to save your work regularly.
Packet Tracer or your PC could crash and you will lose all unsaved work!