This document provides an overview of IOS basics, ICMP, and NAT configuration for Cisco routers. It includes commands for viewing router information and configuration, enabling CDP, checking line status, viewing ICMP codes, and configuring static and dynamic NAT with overload. Static NAT maps internal IP addresses to external IP addresses in a 1-to-1 ratio, while dynamic NAT uses port address translation for a 1-to-many mapping when addresses are limited.

1. IOS Basics, ICMP, & NAT
Chapter 6
Networking Essentials – Eric Vanderburg © 2005

2. Commands

What can be done in user mode…
Show version – displays info on the IOS &
ROM versions, uptime, image file name, &
hardware specs.
 Enable
 Show running-config


3. Global configuration
(configure terminal)

Change the configuration register

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Config-register 0x2100
Last digit is the boot mode

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View what is in flash – show flash
Add boot system commands

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0 – ROM Monitor
1 – RXBoot mode
2 – look in NVRAM for commands & load IOS from flash
Boot system flash cisco-g-1.2315-9
Show ip route

4. CDP (Cisco Discovery
Protocol)
Cdp enable – starts CDP
 No cdp enable – stops CDP
 Cdp timer 60 – sends updates every 60
seconds
 Cdp holdime 100 – keeps cdp info in ram
for 100 seconds


5. Line status
Serial 0 is down, line protocol is down
 First is layer 1, second is layer 2


6. ICMP codes
! – successful
 . – request timed out
 U – destination unreachable
 & - TTL exceeded


Trace uses the TTL to track the route

7. NAT
(Network Address Translation)
Static – 1 to 1, pre-configured
 Dynamic, limited address space
 Dynamic with overload, uses PAT, 1 to
many


8. Static NAT commands

Configure terminal

Interface Ethernet 0

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Interface Serial 0
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ip address 10.1.1.3 255.255.255.0
Ip nat inside
Ip address 200.1.1.251 255.255.255.0
Ip nat outside
Ip nat inside source static 10.1.1.2 200.1.1.2
Ip nat inside source static 10.1.1.1 200.1.1.1

9. Dynamic NAT commands
<uses prior commands minus the inside
source static lines>
 Access list 1 permit 10.1.1.2
 Access list 1 permit 10.1.1.1
 Ip nat pool Test 200.1.1.1 200.1.1.2
netmask 255.255.255.252
 Ip nat inside source list 1 pool Test


10. Dynamic NAT with Overload
commands
<uses prior static commands minus the
inside source static lines & access list
from dynamic>
 Ip nat inside source list 1 interface Serial
0 overload


11. NAT commands
Clear ip nat translation – used in
dynamic to reset all translations
 Show ip nat translations – show which
internal addresses & ports are mapped
to which external addresses & ports
 Debug ip nat – shows each time an
address is translated
