ccna pro

ccna pro.Many protocols run at the network layer, such as AppleTalk. DECnet, TCP/ IPv4. TCP/ IPv6.IPX, Vines.XNS, etc. Each of these protocols has its own method of defining logical addresses. By correctly assigning logical addresses to devices in the network, hierarchical designs can be built that can scale to very large scale. This is where logical addresses are superior to Layer 2 addresses, which use a flat design and are not scalable.All Layer 3 addressing schemes have two parts: the network part and the host (node) part. Within the network, each segment (physical or logical) must have a unique network number; within those segments, each host must have a unique host number, which belongs to the corresponding network number. The network number assigned to the device and the host number together form a unique Layer 3 address throughout the network. For example, if there are 500 devices running TCP/IP in the network, each of these devices must have a unique TCP/IP Layer 3 address.This is different from the Media Access Control (MAC) address used at Layer 2. the MAC address only needs to be unique within the current physical (or logical) network segment. In other words, within the same broadcast domain, all MAC addresses must be unique; however, two different broadcast domains can contain the same MAC address.This address uniqueness is much like the postal system. For example, two houses on the same street cannot have the same house number (e.g., both 1597 BerryStreet), otherwise the mail carrier would not know which house to deliver the mail to. However, if the two street addresses are the same, but the zip codes are different, the mail carrier will be able to distinguish between the two addresses, even if they have the same street name and door number. "MAC addresses and broadcasts are discussed in more depth later in this chapter in the "Layer 2: Data Link Layer" section.To help understand the components of a Layer 3 address, let's look at a few examples.TCP/IPIPv4 addresses are 32 bits long; to make them easier to understand, they are divided into four bytes, and adjacent bytes are separated by periods. This is called the dotted decimal representation. The following is a simple IP address: 101.1. There is also a value called the subnet mask, which determines the boundary between the network portion of the address and the host portion. The TCP/IP addressing scheme seems to be the most complex compared to the addressing schemes of other protocols. IP addressing is covered in detail in Chapter 6.Most of the other protocols have much simpler address formats. For example, an IPX address is 80 bits long, with the first 32 bits being the network number and the last 48 bits being the host address. In the IPX address, the host portion defaults to the MAC address of the network interface card (NIC), but can be modified by users and administrators. Each protocol has its own addressing scheme, but each scheme begins with the network portion and then the host portion.2. Routing TableRouters are devices that run at the network layer and they make routing decisions based on logical network numbers: how to send information (often called packets) to destinations. Routers create routing tables that contain path information, including the network number, the interface that should be used to travel to the specified network number, the metric value of the path (the overhead to the corresponding destination), how the specified network number was learned, and how old or new the information is. The metric value is used to measure the merit of different paths to the same destination. If there are multiple paths to a destination, these paths are ranked based on the metric value to determine which path is the best. After selecting the best route to the destination network based on the metric, the router adds it to the routing table. There are numerous types of metric values used, such as bandwidth, latency, hop count, etc. Each routing protocol has its own metric structure; for example, TCP/IP Routing Information Protocol (RIP) uses hop count, while Cisco Enhanced Interior Gateway Routing Protocol (EIGRP) uses bandwidth, latency, reliability, load, and maximum transmission unit (MTU). Routing and metric values are discussed in Section 4.Upon receiving an inbound packet, the router looks at the Layer 3 address in the packet header, determines the network number in it, and compares it to its own routing entry. If it finds a matching routing entry, the router forwards the packet out the appropriate interface; if it does not find a matching routing entry, it discards the packet.Exam Essentials The router makes routing decisions based on the network number in the Layer 3 address (such as an IP address). The location of the network is stored in the routing table.