Question

The Internet is down.

You ping yourself at 127.0.0.1, and everything is fine on your machine. You ping www.google.com , and it times out. You do an ipconfig /all on your host machine. What can you assume if your ipconfig /all command listed the default gateway as being 0.0.0.0? The router!

As an experienced IT person will tell you, the best thing to do is turn any device off and on again—first your host and then the router. Still not working? Expand your hypothesis to another host on your network—can it reach the Internet or the router? Does it pull an IP address from the router? When you are troubleshooting, it is all about the scientific method. Form a hypothesis, test, modify, and form a new hypothesis.

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Here are two more acronyms to add to your IT vernacular: DHCP and DNS. DHCP stands for Dynamic Host Configuration Protocol. Let's isolate each word.

• Dynamic : Ever‐changing, fluid
• Host : Asset on a network
• Configuration : How the asset is supposed to work
• Protocol : Rules that allow two more assets to talk

DHCP is a network management tool. This is the tool that dynamically assigns an IP address to a host on a network that lets it talk to other hosts. Most simply, a router or a gateway can be used to act as a DHCP server. Most residential routers will get their unique public IP address from their ISP. This is who you write the check to each month.

In a large enterprise, DHCP is configured on servers to handle large networks' IP addressing. DHCP decides which machine gets what IP address and for how long. If your machine is using DHCP, did you notice in your ipconfig /all command how long your lease was? If you are not leasing, then you are using a static IP address.

Here are two more commands for you to use if you want a new IP address:

• ipconfig /release : This releases all IPv4 addresses.
• ipconfig /renew : This retrieves a new IP address, which may take a few moments.

DNS is an acronym for Domain Name System. This is a naming system for all hosts that are connected to the Internet or your private network. As you do what you do on the Internet or in a private network, DNS will remember domain names. It will store this data in something we call a cache (pronounced “cash”). This is done to speed up subsequent requests to the same host. Sometimes your DNS cache can get all wonky—sometimes by accident, sometimes by a hacker.

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NOTE

Cache poisoning—sometimes called DNS spoofing—is an attack where a malicious party corrupts the DNS cache or table, causing the nameserver to return an incorrect IP address and network traffic to be diverted.

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Here are two more commands to try:

• ipconfig /displaydns : This may scroll for a while because this is a record of all the domain names and their IP addresses you have visited on a host.
• ipconfig /flushdns : If you start encountering HTML 404 error codes, you may need to flush your cache clean. This will force your host to query nameservers for the latest and greatest information.