I can't download apache2 and php5:
sudo apt-get install apache2 php5 libapache2-mod-php5
Gives me the error "Failed to fetch [URL] 404 Not Found". The raspberry is connected via ethernet to my FritzBox router, but it isn't displayed as a known device.
I tried to ping google.com and that works, so I'm not sure what the problem is.
Always run sudo apt-get update first before installing / upgrading as that will ensure you are getting the newest packages / package locations.
404 errors are commonly because packages are no longer available as new ones have replaced them, or you aren't connected to the internet. But as you can ping google.com, that rules out internet connectivity issues.
After sudo apt-get update, that's when you run sudo apt-get install apache2 php5 libapache2-mod-php5
