Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

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100 CHAPTER 6 TCP/IP and Routing contain a number of elements in a specified order. Each of those elements is required and each is described briefly here. ■ Network destination The network ID can be class-based, subnetted, or supernetted. ■ Netmask The mask used to match the destination network with the IP address in the data. ■ Next hop or gateway The IP address of the next router (a hop is one segment between routers. If a packet has to go through two routers, that would be two hops). ■ Interface Identifies which network interface is used to forward the packet. Remember that every router has at least two interfaces. ■ Metric The metric is a number used to help determine the best route for the packet. This typically is used to identify the route with the fewest hops. The metric is often expressed as the cost of the route. Routing tables can also store four specific types of routes: ■ Directly attached network IDs For packets destined for the local or attached network. If the sending and receiving hosts are both on the same subnet, for instance, the packet will be sent through this method. ■ Remote network IDs Any packets destined for networks reachable through routers will be sent through this routing method. ■ Host routes A host route is a route to a specific IP address. This type of route allows a packet to be sent to a specific IP address. The network ID is the IP address of the destination host and the network mask is 255.255.255.255. ■ Default route The default route is used when a more specific network ID or route cannot be found. When all else fails, the default route is used. This is defined as a network ID of 0.0.0.0 and the network mask is 0.0.0.0. Route Determination Process Each IP packet has a destination IP address, which is used to determine how the packet will be routed. Using the logical ANDing process, the destination IP address and the subnet mask (or netmask) are compared. If they match, the packet stays on the local network and is sent directly to the destination IP address. If the destination IP address and the subnet mask do not match, the entries in the routing table are compared with the destination IP address. If a match is found (that is, if the destination IP address and the subnet mask AND to a value found in the routing table), the packet is sent to the gateway listed in the routing table. If no matching entries can be found, the packet is sent to the defined default gateway. If more than one match is found in the routing table entries, the metric is used and the route with the fewest hops is typically selected. To view the route table on a Windows Server 2003 computer, access the command prompt, and type route print. If there is no matching entry in the routing table, the packet will be sent to the default gateway for forwarding. When this process is complete, the resulting IP
Understanding Basic IP Routing 101 address (either destination IP address or gateway IP address) is then resolved to a physical address. This process uses ARP. PHYSICAL ADDRESS RESOLUTION (USING ARP) ARP resolves IP addresses to physical addresses. ARP is used to resolve the nexthop IP address to a physical MAC address using network broadcasts. The resolved MAC address is placed in the header of the packet as the destination MAC address. DID YOU KNOW Just as a routing table is stored on the local host, so too is a list of the resolved IP-to-MAC addresses. This information is held in the ARP cache. Each time a request and resolution occur, both the sender and receiver store the other’s IP-to-MAC address mapping. When a packet is received, the ARP cache is checked to see if the resolution has already been added to the cache. If so, the packet is immediately forwarded to the resolved address. ARP process – The two steps in resolving an IP address to a MAC address are the ARP request and ARP reply. The format of the ARP request is a MAC-level broadcast that is sent to all nodes on the same physical segment as the sender. Whichever node sends the ARP request message is called the ARP requester. The node whose address matches the MAC address in the ARP request will respond by sending an ARP reply. This is a unicast (directly back to the sender only) MAC frame sent by the node called the ARP responder. The ARP responder’s unicast message contains both its IP address and its MAC address. ■ ■ ■ Reverse ARP (RARP) will resolve an IP address to a known MAC. Inverse ARP (InARP) is used to resolve the IP address on the other end of a virtual circuit. Proxy ARP occurs when one node answers ARP requests on behalf of another node. An ARP proxy device is often a routing device, but it does not act as an IP router. STATIC AND DYNAMIC IP ROUTERS Routing tables can be updated manually or dynamically. If the table must be updated manually, it is considered to be static. If the table can be updated automatically, it is considered to be dynamic. Dynamic routing occurs when routing tables are automatically and periodically updated, and it relies upon routing protocols. Dynamic routing can be divided into two different classifications: ■ Interior gateway protocol (IGP) is designed for routing within an autonomous system. The three most commonly used IP-based IGP routing protocols, are as follows: ■ Routing information protocol (RIP) is a distance vector routing protocol, and it determines routes based on the number of hops (how many routers
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188 Index Network troubleshooting,
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