1. Troubleshooting IP Addressing
1. Open a DOS window and ping
This is the diagnostic or loopback address
If successful, IP stack is then considered to be initialized
If it fails, then you have an IP stack failure and need to reinstall TCP/IP on the host
2. From the DOS window, ping the IP address of the local host
If that’s successful, then your Network Interface Card (NIC) card is functioning
If it fails, then there is a problem with the NIC card

2. Troubleshooting IP Addressing
3. From the DOS window, ping the default gateway (router)
If the ping works, it means that the NIC is plugged into the network and can communicate on the local network
If it fails, then you have a local physical network problem that could be happening anywhere from the NIC to the router
4. If steps 1 through 3 were successful, try to ping the remote server
If that works, then you know that you have IP communication between the local host and the remote server also that the remote physical network is working

3. Determining IP Address Problems
It’s common for a host, router, or other network device to be configured with the wrong IP address, subnet mask, or default gateway
First step is to draw the whole network and IP addressing scheme
For example take a look on the following case

4. Determining IP Address Problems
A user in the Sales department calls and tells you that he can’t get to ServerA in the Marketing department
You ask him if he can get to ServerB in the Marketing department, but he doesn’t know because he doesn’t have rights to log on to that server. What do you do?

5. Determining IP Address Problems
You ask the client to go through the four troubleshooting steps that we learned
Steps 1 through 3 work, but step 4 fails
By looking at the figure, can you determine the problem?
First, the WAN link between the Lab_A router and the Lab_B router shows the mask as a /27
You should already know this mask is and then determine that all networks are using this mask

6. Determining IP Address Problems
The network address is What are our valid subnets and hosts? 256 – 224 = 32
So this makes our subnets 32, 64, 96, 128, etc
So, by looking at the figure, you can see that subnet 32 is being used by the Sales department, the WAN link is using subnet 96, and the Marketing department is using subnet 64

7. Determining IP Address Problems
Now you’ve got to determine what the valid host ranges are for each subnet
The valid hosts for the Sales LAN are 33 through 62—the broadcast address is 63 because the next subnet is 64, right?
For the Marketing LAN, the valid hosts are 65 through 94 (broadcast 95), and for the WAN link, 97 through 126 (broadcast 127)
By looking at the figure, you can determine that the default gateway on the Lab_B router is incorrect
That address is the broadcast address of the 64 subnet so there’s no way it could be a valid host

8. Optimization & Troubleshooting
IPCONFIG
Entered fro the command prompt and it displays the IP address information for the local host
PING
One of the most commonly used tool for troubleshooting the TCP/IP problems
ROUTE
Displays the local routing table
TRACERT
Informs about the health of the route
Shows the hops, and how long it took to get from one router to another

9. Optimization & Troubleshooting
NETSTAT
Displays the TCP/IP protocol statistics
NBTSTAT
Displays the current NETBIOS name cache
Event viewer
Windows based tool. Whenever TCP/IP has a problem initializing, an error message displays and an entry is logged in the system event log.

10. Routing Basics
The term routing is used for taking a packet from one device and sending it through the network to another device on a different network
Routers don’t really care about hosts—they only care about networks and the best path to each network
The logical network address of the destination host is used to get packets to a network through a routed network
Then the hardware address of the host is used to deliver the packet from a router to the correct destination host

11. Routing Basics
To be able to route packets, a router must know, at a minimum, the following:
Destination address
Neighbor routers from which it can learn about remote networks
Possible routes to all remote networks
The best route to each remote network
How to maintain and verify routing information

12. Routing Basics
The router learns about remote networks from neighbor routers or from an administrator
The router then builds a routing table that describes how to find the remote networks
If a network is directly connected, then the router already knows how to get to it
If a network isn’t connected, the router must learn how to get to the remote network in two ways:
by using static routing, meaning that someone must hand-type all network locations into the routing table
or through something called dynamic routing

13. Routing Basics
In dynamic routing , a protocol on one router communicates with the same protocol running on neighbor routers
The routers then update each other about all the networks they know about and place this information into the routing table
If a change occurs in the network, the dynamic routing protocols automatically inform all routers about the event
If static routing is used, the administrator is responsible for updating all changes by hand into all routers
Typically, in a large network, a combination of both dynamic and static routing is used

14. The IP Routing Process

15. The IP Routing Process
A router receives the frame and checks the received frame check sequence (FCS); if errors occurred, the frame is discarded. The router makes no attempt to recover the lost packet.
If no errors occurred, the router checks the Ethernet Type field for the packet type, and extracts the packet. The Data Link header and trailer can now be discarded.
Assuming an IP packet, the router checks its IP routing table for the most specific prefix match of the packet’s destination IP address

16. The IP Routing Process
The matched routing table entry includes the outgoing interface and next-hop router; this information points the router to the adjacency information needed to build a new Data Link frame.
Before creating a new frame, the router updates the IP header TTL field, requiring a re-computation of the IP header checksum
The router encapsulates the IP packet in a new Data Link header (including the destination address) and trailer (including a new FCS) to create a new frame

17. Routing Protocol Basics
Administrative Distances
An administrative distance is an integer from 0 to 255
The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router
0 is the most trusted and 255 means no traffic will be passed via this route.

18. Administrative Distance (AD)

19. Routing Protocol Basics
If a router receives two updates listing the same remote network, the first thing the router checks is the AD
If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table
If both advertised routes to the same network have the same AD, then routing protocol metrics (such as hop count or bandwidth of the lines) will be used to find the best path to the remote network
The advertised route with the lowest metric will be placed in the routing table.

20. Routing Protocol Basics
If a network is directly connected, the router will always use the interface connected to the network
If an administrator configures a static route, the router will believe that route over any other learned routes
You can change the administrative distance of static routes, but, by default, they have an AD of 1

21. Routing Protocols There are three classes of routing protocols:
Distance vector
Link state
Hybrid

22. Distance vector Protocols
The distance-vector protocols find the best path to a remote network by judging distance
Each time a packet goes through a router, that’s called a hop
The route with the least number of hops to the network is determined to be the best route
The vector indicates the direction to the remote network
RIP and IGRP are examples of distance-vector routing protocols
They send the entire routing table to directly connected neighbors

23. Link-state Protocols
In link-state protocols, also called shortest-path-first protocols, the routers each create three separate tables
One of these tables keeps track of directly attached neighbors, one determines the topology of the entire internetwork, and one is used as the routing table
Link-state routers know more about the internetwork than any distance-vector routing protocol
OSPF is an example of a completely link state IP routing protocol
Link state protocols send updates containing the state of their own links to all other routers on the network

24. Hybrid Protocols
Hybrid protocols use aspects of both distance vector and link state
EIGRP (Enhanced Interior Gateway Routing Protocol) is an example