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The current TCP/IP network, all are connected through a router, Internet is tens of thousands of IP subnet via a router interconnected international network. This network is called router-based network (router based network), formed the router node of "NET NET". "NET NET", the router is responsible not only for IP packet forwarding, will also be responsible for liaison with other routers, to determine "NET NET" Routing and maintains a routing table.
when IP subnet a host sends the IP packet to another host in the same IP subnet, it will directly send the IP packet to the network, the other routers will receive. And for different IP on the Internet host, it selects a destination subnet, IP packet to the router, the router is responsible for the IP packet to the destination. If you do not find such a router, host IP packet was sent to a group called "default gateway (default gateway)" on the router.
"default gateway" is a configuration parameter for each host, which is a router connected to the same network port IP address.
router when forwarding an IP packet, grouped by IP-only destination network number portion of the IP address, select the appropriate port and sends the IP packet. As with the host, received by the router to determine the port of the destination subnet, if it is, directly through the send port group to the network, or to select the next router to send packet. The default gateway of the router has its for transferring don't know where to send the IP packet. In this way, through the router knows how to transmit IP packet forwarding correctly, do not know the IP packet to the "default gateway" routers, so level-level transmission, IP Group will eventually be sent to the destination, send no destination IP packet is discarded by the network.
route includes two basic elements: Routing and forwarding.
routing: determine the best path to the destination, by routing algorithms to achieve. Because of different routing protocols and routing algorithms, are relatively complex. In order to determine the best path routing algorithm must initiate and maintain routing table contains routing information, routing information is dependent on the routing algorithm is different. Routing algorithm will collect different kinds of information to fill in the routing table, according to the routing table can be a destination network and the next station (nexthop) tells the router.
between routers to Exchange routing information updates, update maintains a routing table to reflect the changes in the network topology, and according to measurements by the router to determine the best path. This is the Routing Protocol (routing protocol), such as routing information Protocol (RIP) and open shortest path first (OSPF) and border Gateway Protocol (BGP).
forwards: along the path-finding the best path routing information packet. First look in the routing table of the router, and identify whether or not know how to send packet to the next site (router or host), if the router does not know how to send a packet, often use this packet; otherwise according to the routing table of the corresponding table entry to send the group to the next site, if the purpose of the network is directly connected to the router, the router directly to the corresponding port on the group. Forward
routing protocols and routing protocols are independent concepts based on mutual cooperation, the former with the latter routing table maintenance, to take advantage of the functionality provided by the former while the latter to distribute routing protocol packets.