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In this chapter, you learned that networks must be designed to support the benefits of advanced IP routing protocols. Well-designed networks allow corporations to react quickly to changes in their networking requirements, including mergers, reorganizations, and downsizing.

There are two types of hierarchical network design: functional and geographic. In a functional network design, the different divisions of a corporation have their own networks and are connected according to their functional purpose within the corporate structure. In a geographic network design, the divisions of a corporation have their own networks and are connected according to their location.

The access, distribution, and core layers comprise a hierarchical scalable network design.

In a fully meshed core layer design, all routers in the core have direct connections to all other routers in the core. A core layer hub-and-spoke configuration establishes a focal point for the data flow at a key site.

A good IP addressing plan implemented in a well-designed network provides scalability, predictability, and flexibility.

RFC 1918 has set aside the following IPv4 address space for private use:

  • Class A network— to

  • Class B network— to

  • Class C network— to

The benefits of hierarchical addressing include a reduced number of routing table entries and efficient allocation of addresses.

A subnet mask is a 32-bit value that identifies which bits in an address represent network bits and which represent host bits. To create a subnet mask for an address, use a 1 for each bit of the address that you want to represent the network or subnet portion of the address, and use a 0 for each bit of the address that you want to represent the node portion of the address. The number of subnetworks created by adding n bits to the default mask is calculated by the formula 2n. The number of hosts available is calculated by the formula 2n – 2, where n is the number of bits in the host portion.

A prefix is a slash (/) followed by a numeric value that is the number of bits in the network and subnet portions of the address—in other words, the number of contiguous 1s that would be in the subnet mask.

A major network is a Class A, B, or C network. With classful routing, routing updates do not carry the subnet mask. Therefore, only one subnet mask must be in use within a major network; this is known as FLSM. With classless routing, routing updates do carry the subnet mask. Therefore, different masks may be used for different subnets within a major network; this is known as VLSM.

With VLSM, it is important to remember that only unused subnets should be further subnetted. In other words, if you use any addresses from a subnet, that subnet should not be further subnetted.

Route summarization (also called route aggregation or supernetting) can reduce the number of routes that a router must maintain, because it is a method of representing a series of network numbers in a single summary address. Route summarization is most effective within a subnetted environment when the network addresses are in contiguous blocks in powers of 2.

Routing information advertised out an interface is automatically summarized at major (classful) network address boundaries by RIP, IGRP, and EIGRP. When using EIGRP and RIPv2, you can disable this automatic summarization. For OSPF and IS-IS, you must configure summarization.

CIDR is a mechanism developed to help alleviate the problem of exhaustion of IP addresses and growth of routing tables. The idea behind CIDR is that blocks of multiple addresses (for example, blocks of Class C address) can be combined, or aggregated, to create a larger classless set of IP addresses, with more hosts allowed.

The difference between CIDR and route summarization is that route summarization is generally done within, or up to, a classful boundary, whereas CIDR combines several classful networks.

NAT terminology includes the following:

  • Inside local IP address—The IP address used by the inside host before any translations.

  • Inside global IP address—The IP address used by the inside host as it appears to the outside network; this is the translated IP address.

  • Outside global IP address—The configured globally unique IP address assigned to a host in the outside network.

  • Outside local IP address—The IP address of an outside host as it appears to the inside network.

When you use only access lists for NAT, the resulting NAT table has only simple translation entries, identifying only which inside local address is being translated to which inside global address. To get an extended translation entry in the NAT table, you must either configure NAT for overloading or use route maps.

Route maps are complex access lists that allow some conditions to be tested against a packet or route in question using match commands. If the conditions match, some actions can be taken to modify attributes of the packet or route. These actions are specified by set commands.

IPv6 addresses have 128 bits. The IPv6 header has 40 octets in contrast to the 20 octets in IPv4. IPv6 has a smaller number of fields, and the header is 64-bit aligned to enable fast processing by current processors. The IPv6 address fields are four times larger than in IPv4.

IPv6 addresses are represented as a series of 16-bit hexadecimal fields separated by colons (:), in the format x:x:x:x:x:x:x:x. The leading 0s within a field are optional. Two colons (::) may be used to compress successive hexadecimal fields of 0s. This can be done at the beginning, middle, or end of an IPv6 address, but it is allowed only once in an address.

IPv6 addresses can be unicast (one-to-one), anycast (one-to-nearest), or multicast (one-to-many); IPv6 has no concept of a broadcast address.

With IPv6 stateless autoconfiguration, a router on the local link sends network-type information to all its nodes. An IPv6-enabled host uses the prefix advertised by the router as the top 64 bits of the address; the remaining 64 bits contain the 48-bit MAC address in an extended universal identifier 64-bit (EUI-64) format. This autoconfiguration produces a full 128-bit address that can be used on the local link and that guarantees global uniqueness.

The two most common techniques to transition from IPv4 to IPv6 are as follows:

  • Dual stack—IPv4 and IPv6 stacks run on a system. The system can communicate with both IPv4 devices and IPv6 devices.

  • Tunneling—The most common type of tunneling used is IPv6 to IPv4 (6to4) tunneling, to encapsulate IPv6 packets in IPv4 packets. Each 6to4 edge router has an IPv6 address with a /48 prefix, which is the concatenation of 2002::/16 and the edge router's IPv4 address (in hexadecimal). The edge routers automatically build the tunnel using the IPv4 addresses imbedded in the IPv6 addresses.

The following routing protocols or draft proposals are available for IPv6: RIPng, OSPFv3, IS-ISv6, and BGP4+.

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