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Unformatted text preview: to a network, and the destination is a network address: a net ID, or a
combination of a net ID and a subnet ID.
D The route was created by a redirect (Section 9.5).
M The route was modified by a redirect (Section 9.5).
The G flag is important because it differentiates between an indirect route and a direct
route. (The G flag is not set for a direct route.) The difference is that a packet going out a
direct route has both the IP address and the link-layer address specifying the destination
(Figure 3.3). When a packet is sent out an indirect route, the IP address specifies the final
destination but the link-layer address specifies the gateway (that is, the next-hop router).
We saw an example of this in Figure 3.4. In this routing table example we have an indirect
route (the G flag is set) so the IP address of a packet using this route is the final destination
(22.214.171.124), but the link-layer address must correspond to the router 126.96.36.199.
It's important to understand the difference between the G and H flags. The G flag file:///D|/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/ip_rout.htm (3 of 17) [12/09/2001 14.46.50] Chapter 9. IP Routing differentiates between a direct and an indirect route, as described above. The H flag,
however, specifies that the destination address (the first column of netstat output) is a
complete host address. The absence of the H flag means the destination address is a network
address (the host ID portion will be 0). When the routing table is searched for a route to a
destination IP address, a host address entry must match the destination address completely,
while a network address only needs to match the network ID and any subnet ID of the
destination address. Also, some versions of the netstat command print all the host
entries first, followed by the network entries.
The reference count column gives the number of active uses for each route. A connectionoriented protocol such as TCP holds on to a route while the connection is established. If we
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12