because the operating system can't see what's going on inside the connection or even know about the semantics of individual elements, the user-agent does that.

because the operating system can't see what's going on inside the connection or even know about the semantics of individual elements, the user-agent does that. the value of dns/ip level blocking is diminishing as resource density per host increases due to ipv4 shortage and should be completely inpractical with ipv6 due to adress space size.