Analysis of routers, routes, and route tables

Analysis of routers, routes, and route tables
A router is a network device. It is equipped with multiple network interface cards (NICS) and can use its network knowledge to correctly forward incoming traffic. Deciding whether an entry packet should be sent to the local host or the Information required for Forwarding, and the Information required for correct packet Forwarding during Forwarding, is stored in a Forwarding Information Base, in a database, it is usually called a route table. ISP, the basic element of a route, provides the forwarding information library of companies or organizations that access the internet, if the route from host A to host B is the same as the route from host B to host A, this route is called A symmetric route. However, in the case of complex configuration, the returned routes may be different, which is called asymmetric routing. Metrics is an optional parameter configured on a route. Realm, a numerical representation of the domain name www.2cto.com address classification, IP address is divided into different classes. The IP address specification specifies some IP addresses in a specific range as non-routing addresses, which means these addresses can only be used in one LAN. The routing address must be processed by the Central Body and globally unique. Correspondingly, anyone can configure non-route addresses, but most of these addresses are used in the system behind the vro. Non-route addresses cannot be used to provide any Internet services, because they are not the only one in the world, and the Internet Router cannot send traffic to them. Scope, routing, and IP address can both specify a scope to tell the kernel under which conditions they are meaningful. In Linux, the scope of the route indicates the distance to the destination network, and the scope of the IP Address indicates how far the IP address is from the local host, to some extent, it also tells you how far the address owner is from the local host. The IP address is commonly used as the scope Host. When an address is only used for internal communication of the Host, its scope is the Host. The address is unknown outside the host and cannot be used for external communication. For example, 127.0.0.1 Link, when an address is only valid within a LAN (that is, a network connected by each computer through the Link layer) and used only within the LAN, the scope of this address is a link. For example, the broadcast address of a subnet. Packets sent from one host in the subnet to the subnet broadcast address are sent to other hosts in the same subnet.
Global. When an address can be used anywhere, its scope is global. This is the default scope for most addresses. Scope cannot reflect the differences between non-routable (private) addresses and public addresses. The scope of 10.0.0.1 and 165.12.12.1 can be a link or a global domain. You can specify the scope when the system administrator configures the address. Because global scope is the default value for the two IP addresses mentioned above, if you want the two addresses to be different, the Administrator must specify a scope. The kernel automatically sets the appropriate scope value for the broadcast address and loop address. The scope www.2cto.com host of the route address. When the destination address of a route is a local host, its scope is the host link. When the destination address of a route is a local network, its scope is the link global. When the destination address indicated by a route exceeds one million hops, its scope is the global. The default gateway is often used as the 0.0.0.0/0 route. A host connected to the Internet is usually configured with a router to reach the local LAN, and is also the default router for the internet. A targeted broadcast packet is a simple packet sent to a subnet broadcast address. Subnet broadcast is generally initiated by a host in the same subnet. This means that the destination of the broadcast packet is the host in its own subnet. On the other hand, the destination of the targeted broadcast is the broadcast address of the remote subnet. A Master Address and a secondary address run multiple services on a host, and each service requires a different IP address. This method can also simplify firewall rules. The two subnets may be temporarily merged to a hub or switch due to lack of hardware. This is a NIC that can connect the two subnets. When an address is configured, if the address is in the same subnet as the address already configured on the same NIC, the address is considered as a secondary address, which includes the same subnet. Therefore, the order of Address Configuration is very important: although the IP address is not explicitly specified as the primary address or secondary address, it can be determined based on the existing address and subnet mask. In short, the primary address or secondary address is not only an IP address, but also a subnet mask that marks the subnet. When configuring multiple IP addresses on an interface, it is important to understand the difference between the primary address and the secondary address. The route table www.2cto.com uses two route tables in Linux by default: one table is used for the local address. A successful query from this table indicates that the packet is to be submitted to the Host. A table is used for all other routes. The data items can be manually configured or dynamically inserted using the routing protocol. The default route action is based on the route forwarding information obtained from the route table, that is, the next hop router address and the egress device forward the packet black hole, packets that match this type of routing will be quietly discarded. Inaccessible. packets that match this type of routing are discarded, and a message that is not reachable on the icmp Host is generated. packets that match this type of routing are discarded, and generate an icmp packet filtered message to quit. This type of routing is used together with the Policy Routing. I am * Li Shimin *