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How can the Network layer know the destination IP?

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DNS

When you put the url in the browser, a number of thngs happen, but basically the browser sends a query out to your local Domain Name servers and they look to see if there is an A record which is what translates the domain name to an IP address. If one isn't found then it goes to the next level which is your ISP and does the same check. This keeps going until it hits the root level servers (the brains to the internet so to speak). Once it has this IP address then it goes on to the network which owns the address and the process is somewhat reversed, but not quite. It all depends on the network on the receiving end. It may go through a firewall or proxy NAT (network address translation) which converts the outward facing IP to an internal IP address, but in essence the request is sent on to an internal server and the webserver then takes that URL and port and serves up the appropriate web page.