To best understand them, it's helpful to understand what DNS is and how it works. DNS stands for domain name system, and it basically acts like your contact list in your phone. You click on Bob, the phone dials Bob's phone number. Everything in a network works on IP addresses, and DNS translates the easy to remember domain name google.com to the less easy 172.217.11.46 that's the actual IP address that listens to requests directed to google.com.
I'm explaining this from memory, so forgive me if I get something slightly out of order, but in a nutshell, the order of operations is something like this:
1. The local hosts file (c:windows/system32/drivers/etc/hosts) is checked, and returned if found. (This is useful for example when the resource you are trying to access is on your internal network, so you want to use that IP address instead of the public IP address returned to the rest of the world that is not inside your firewall.)
2. The local DNS cache is checked. If the answer is present and still considered fresh, it gets used.
3. The primary DNS server configured for the network adapter is queried. It either returns a fresh cached entry or queries upstream.
The most likely answer for why a small number of sites didn't work would probably be one of two things. Either your DNS resolution was not working and those sites' cache entries were stale, or DNS resolution was working, but those sites' IP addresses had changed, but your cached resolutions were now wrong, but were not yet stale.
= new reply since forum marked as read
