No matter how long you’ve been working on networks, it never
hurts to review the basics every now and then—even seasoned administrators
sometimes need a refresher. Taking a break from your well-honed routine of
shortcuts and going over the finer points of networking can even open your eyes
to new best practices and troubleshooting methods.

Last time, we discussed five things everyone should know about
Cisco networking and working with routers, including the difference between
a router and a switch, the basics of IP addressing, and more. Now, let’s look
at five more things you should know about Cisco networking.

Comprehend the difference between routing and routed protocols

It always surprises me how many people don’t really know
what a routing protocol is or why it’s necessary. And there are those who can’t
explain the difference between a routing
protocol and a routed protocol.

Open Shortest Path First (OSPF), Routing
Information Protocol (RIP), Enhanced Interior
Gateway Routing Protocol (EIGRP), and BGP are all routing protocols. They distribute routing information throughout
all routers on a network. Each router must know which networks all others
routers have connected to, which enables the router to determine the best path
to take to deliver traffic.

The Internet Protocol (IP) is a routed protocol. Conversely, NetBIOS Extended User Interface
(NetBEUI) is an example of a non-routed protocol.

For more information, see “Cisco
administration 101: What you need to know about EIGRP” and “Take a
closer look at routing redistribution.”

Be familiar with Cisco’s administrative modes

The Cisco IOS isn’t always user-friendly or intuitive. That’s
why it’s important for admins to understand the varying levels of privilege on
a router. Understanding User EXEC Mode, Privileged EXEC Mode, and Global Configuration
Mode is a definite “must-know.”

User EXEC Mode: Designated
by router>, this is the default mode when you first log in to a Cisco
router. However, you can’t do very much in this mode; you have access to some
information, but you can’t make any changes. To move to Privileged EXEC Mode,
type enable.

Privileged EXEC Mode:
Designated by router#, this mode usually has full administrative access on the
router to view pretty much anything. For example, you can use the show command when in this mode. However,
you still can’t make any changes; for that, you need to be in Global
Configuration Mode. To do so, type config
terminal.

Global Configuration Mode:
Designated by router(config)#, this mode offers full access to make changes to
the router’s configuration. To move back to Privileged EXEC Mode, type exit.

For more information, see “Understand
the levels of privilege in the Cisco IOS.”

Know the various uses of the show command

Cisco’s show
command offers a wide variety of uses. While it’s important that you have a
general understanding of all these uses, some are more useful than others. In
my opinion, here are the three most helpful uses of the show command.

Show ip route

This command displays the available routes on the router.
These routes can be static or dynamic. If the destination that your traffic is
going to isn’t on the list and there’s no default route, the router will drop
(throw away) that traffic.

Show running-configuration

This command displays the current configuration of the
router. If you make changes to the configuration, make sure you save those
changes by using the copy run start
command.

Show ip interface brief

This command displays a summary of the current status of all
interfaces on the router. You can use this output to see how many interfaces
are on the router, their types, the IP address of the interfaces, and if the
interface is up or down.

For more information, check out “Effectively
filter Cisco router command output” and “Take
advantage of Cisco’s show version command.”

Understand how private IP addressing and NAT work together

Based on RFC
1918, private IP addresses are not routable on the Internet. Instead, the
purpose of these addresses is for use on internal networks only.Internet routers just throw away any traffic
received from these networks, which include:

10.0.0.0 /8 or 255.0.0.0
172.16.0.0 /12 or 255.240.0.0
192.168.0.0 /16 or 255.255.0.0

Most companies and internal home networks currently use
private IP addressing. In addition, there’s no need for these networks to be
unique when using private IP addressing.

However, if you use private IP addressing and if Internet
routers throw away traffic from your IP address range, how can you communicate
on the Internet? The answer is network address translation (NAT). NAT
translates private IP addresses into public IP addresses.

For example, let’s say your ISP gives you one public
Internet IP address on your home network. On your internal LAN, you’re using
the 192.168.0.0 /16 network. Your router then uses NAT to translate the internal
IP addresses into your one external IP address. When responses come back, NAT
converts it back into an internal private IP address.

For more information, see “Set up NAT
using the Cisco IOS” and “Cisco IP
subnetting 101: Five more things you should know.”

Know how to troubleshoot a network problem using the OSI model

Troubleshooting a problem is how most network admins earn
their keep. Effective troubleshooting in a jam can save the day and prove your
worth.

While there are several approaches to network
troubleshooting, I recommend starting at the physical layer (Layer 1) of the
OSI model and working your way through each layer until you find the problem.
I’ve found this to be the safest and most straightforward way to solve any
network problem.

For more information, check out “Choose a network
troubleshooting methodology” and “Get
real-world examples for applying network troubleshooting methodologies.”

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David Davis has worked
in the IT industry for 12 years and holds several certifications, including
CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of
systems/network administrators for a privately owned retail company and
performs networking/systems consulting on a part-time basis.