10.1.2.0/24, the packet would get to Router B. However, if the Web server is down, Router B
will not get an ARP reply from Web; Router B will send an unreachable back to Fred, with the
Host Unreachable code field.
Router B needed to fragment the packet, but the Do Not Fragment bit was set in the IP header,
the router would send an unreachable back to Fred with the Can't Fragment value in the code
field.
One implies that the protocol above IP, typically TCP or UDP, is not running on that host. This
is highly unlikely today. If true, this host would reply with an unreachable with the code field
value implying Protocol Unavailable. The final code field value is more likely today. If the
server was up but the Web server software was not running, the TCP/IP code on the server
would reply with an unreachable with the code field implying Port Unavailable. In other words,
the Web server software has not opened a listening socket connection using the Web server
well-known port of 80.
value as a separate message. For example, the documentation lists ping responses, stating
something like an "ICMP host unreachable" message. There is no such message, but there is a
"destination unreachable" with the Host Unreachable code set. The exam probably will not
expect you to know that there is no Host Unreachable message, though.
Code
By...
Unreachable
packet.
destination subnet, but the host is not responding.