400 Bad request 
Something is wrong with the URL you entered. It could be that the page you are looking for does not exist, or the server you are contacting does not recognize the URL. Check your typing, especially special characters like slashes and colons as well as uppercase letters. 
401 Unauthorized 
The page is password-protected or does not accept visitors from your domain. If you entered a password, you may have typed it incorrectly. 

403 Forbidden 
Same as 401 Unauthorized. You should try your password again. 

404 Not Found 
This one's common. The page doesn't exist, or exists at a different URL. Try deleting everything after the last slash in the URL to see if you get a page with a link to the page you want. 
Also, check your spelling. And if the URL ends in .htm, type .html instead (and vice versa). Sometimes, however, pages are just missing. 

503 Service Unavailable 
Like a busy signal. Wait a minute and try the page again — chances are, it's there. 

Host Unknown 
The site may have moved. Try looking for it with a search engine (or, if the URL you had was long and complicated, guess how it may have been simplified). Also, check to see if you lost your 
Internet connection (this happens most often if you've gone away from the computer for a while); if you aren't connected, use your dialer to reconnect and retry the site. 

Host unavailable 
Most likely, the host machine is down temporarily for maintenance. Wait a bit and try again. 

Unable to locate host or Unable to locate server 
Similar to Host Unknown; check to see if you may have disconnected from the Internet. If you have, log back on and try again. 

Bad file request 
This is a forms error. Chances are that either your browser does not support the type of form you are trying to use, or there is an error in the form. 

Too Many Connections 
Like a busy signal. Try clicking the Refresh/Reload button, or wait a while and try again. 

Connection refused by host 
Similar to 401 and 403; the page is restricted to certain domains or password-protected. If you think you should have a working password, try entering it again. 

Failed DNS lookup 
This common error means that the Domain Name System (the way in which domain names, like earthlink.net, are translated into official Internet addresses, or IP numbers) could not find an IP address for the URL you entered. Hitting Reload/Refresh sometimes resolves this problem; if not, check to see if you entered the correct URL. 

Helper application not found 
You are requesting to view a kind of file that your browser does not recognize (a graphic format, multimedia, or compressed file). You probably need to install a helper application or plug-in that extends the capabilities of your browser.