My isp or government has blocked access to porn sites but from the same wifi network when I change the dns to Google dns(8.8.8.8) all the blocked sites starts working.
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The Domain Name System is kind of like a phone book of the Internet. Users want to use words and names to refer to services and will enter the domain name of the service they want to use. But the computers will instead of numbers and therefore require the numbered Internet address of the service. So when a user enters a domain the computer use DNS to find the IP address of the service and then send the request to that address.
In a lot of cases it is inconvenient to configure network settings on all computers in a network. They need things like an address, a gateway, netmask, etc. And they need to know the address of the DNS server to use. In order to configure all this we use DHCP. When a computer connects to a network it will send out a DHCP request asking for the network settings. This request never leaves the local network and usually the router is configured to answer these DHCP requests.
Most ISPs set up their own DNS servers in the routers they ship. And they provide the address of their DNS servers in the configuration guide for those who set up their own routers. A common way to block traffic in a relatively soft way is to refuse to serve DNS responses for those domains. This is kind of like providing phone books with certain entries removed. You can still send traffic to the addresses, it is just that you can not look up the address if you only have the domain name.
It have become common for governments to require that ISPs do block certain entries in their DNS servers. This is most common with child pornography sites but some governments require this for distributors of copyrighted material as well. I have not yet heard of legal porn being blocked though but it would not surprise me. This is however a relatively soft way to block traffic. A lot of these filters even redirect you to a site explaining why the content is blocked, although this does not work well with HTTPS. It is seen as a way to prevent accidental exposure to certain content. But it is easy to circumvent if you know how to, or know how to look it up.
A select few authoritarian governments actually block traffic, not just the DNS requests. This is much harder to circumvent but not impossible.
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