1. Troubleshooting IP Communications
Computer Networking
Troubleshooting IP Communications

2. Identifying Scope
One of the first things you’ll need to do when trying to troubleshoot a networking problem is going to be to identify the scope of the problem (Not DNS Scope)
We need to know if the issue is isolated to a single computer, all the computers on a single hub or router, or if the internal network works but there is no connection to the outside internet
Knowing where the problem is will be the first step to solving it

3. Example #1
A user is unable to access a website from his computer. What can we test, and what would it mean?

4. Example #1 Tests Try to access the website from the user's computer
This will tell you if it was a momentary issue or something more. If it is fixed, make a note of it to catch if this is a chronic issue
Try to access a different website from the user’s computer
This will let you know if it is an issue with the website itself, and not us
Try to access the website from another computer
This will tell you if it is a problem localized to their computer
Try to access the website by pinging the IP address
This will catch errors with the DNS for this computer
Try to ping a local node
This will tell you if there is an issue with the local network infrastructure.

5. Ping
Ping sends an ICMP (Internet Control Message Protocol) echo request packet to a specific address. When a host receives that, it will reply with an ICMP echo response
ICMP is an support protocol found in TCP/IP designed for diagnostics
Ping sends 4 packets in order to get an average case response
Ping output includes
IP Address of responder
Size of packet sent
Time from sending the request to getting the reply
Total TTL (Time-to-Live) of the sent packet
Ping will let us know 2 things:
If we have a physical connection to the host
If we have our protocols correctly configured

6. Ping Additional Info
We can ping a DNS name or an IP address. This can help us test DNS servers
Ping pings the local host. This allows us to check to see if our protocols are correctly working without worrying about having a physical connection to a network
Note that some servers and routers may not respond to pings as a security measure to prevent DOS attacks

7. Tracert
Stands for trace route. This is the command for Windows. In linux we would use traceroute
This command shows the path that messages go through to get to a host
Tracert uses TTL in order to see what each step in the process is
TTL (Time-To-Live) is a parameter each packet has
Every time the packet passes from one router to the next, the TTL decrements by one
If the TTL hits zero, the router sends an ICMP time-exceeded error message back to the original sender to let the sender know that the message is lost and not progressing through the network
Tracert uses this by sending a packet with a TTL of 0, then 1, then 2, etc. Each time-exceeded message will contain the IP address of the responding router
Any step that says request timed out indicates that the router is configured to not respond to ICMP requests, just like ping

8. ARP
Allows you to view and edit the ARP table, where MAC addresses are stored in relation to IP address
We attain these MAC addresses from ARP (address resolution Protocol) to request MAC addresses from IP Addresses
These address relations can become invalid in two ways:
The IP Address of the device may change due to DHCP
The MAC address of the device would change if you replace the NIC

9. ARP modifiers
Arp -a displays the ARP table. There will be up to two different types of entries
Static entries are manually entered
Dynamic entries are generated by polling with the ARP protocol
Arp -d (IP ADDRESS) will remove the entry for the given IP address, which we would use if the entry becomes invalid. It requires us to have administrator access. We could then have it dynamically restore the entry by pinging the IP address
Arp -s (IP ADDRESS) (MAC ADDRESS) would allow me to manually create an entry linking the given IP address and MAC address. This will generate a static ARP table entry