Sometimes you type a URL into your browser: examplecat.com/dog.png

but end up at a slightly different URL: examplecat.com/cat.png
ooh, where did the cat come from? I didn’t type that!

Here’s what’s going on behind the scenes:

browser:

GET /dog.png   
HTTP/1.1 Host: examplecat.com

server:

301 Moved Permanently 
Location: /cat.png

browser: okay, I’ll try /cat.png instead

browser:

GET /cat.png HTTP/1.1 
Host: examplecat.com

server:

200 OK
<rest of website here>

The Location header tells the browser what new URL to use.

The new URL doesn’t have to be on the same domain: examplecat.com/panda can redirect to pandas.com. Setting up redirects is a great thing to do if you move your site to a new domain!

! Warning !

301 Moved Permanently redirects are PERMANENT: after a browser sees one once, it’ll always use examplecat.com/cat.png when someone types examplecat.com/dog.png forever. You can’t take it back and decide to not to redirect. If you’re not sure you want to redirect your site for eternity, use 302 Found to redirect instead.