Troubleshooting IP Communications Computer Networking Troubleshooting IP Communications

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

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

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.

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

Ping Additional Info We can ping a DNS name or an IP address. This can help us test DNS servers Ping 127.0.0.1 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

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

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

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