IP Addressing and Subnetting

With classful addressing, class A, B, and C networks can be identified as such by their first several bits or by the range of decimal values for their first octets.

Classless IP addressing means that class A, B, and C rules are ignored. Each address is viewed as a two-part address, formally called the  prefix  and the host parts of the address.

One reserved IP address in each subnet is the subnet number; it has binary 0s for all host bits. This number represents the subnet, and is typically seen in routing tables. The other reserved IP address in the subnet is the subnet broadcast address which is used as a destination IP address to send a packet to all hosts in the subnet.  

Although there are no truly reserved values for the subnet numbers, two (lowest and highest values) can be discouraged from use in some cases:
   -Zero subnet:The subnet fi eld is all binary 0s; in decimal, each zero subnet is the exact same dotted-decimal number as the classful network number, potentially causing confusion.  
   -Broadcast subnet:The subnet fi eld is all binary 1s; in decimal, this subnet’s broadcast address is the same as the network-wide broadcast address, potentially causing confusion.    
In Cisco routers, by default, zero subnets and broadcast subnets work fine. You can disable the use of the zero subnet with the no ip subnet-zero global command. The only time that using the zero subnet typically causes problems is when classful routing protocols are used.  

Classless Interdomain Routing  

The primary goal of CIDR is to improve the scalability of Internet routers’ routing tables. Technically, CIDR uses route summarization, but with Internet scale in mind. CIDR itself မefers to the administrative assignment of large address blocks, and the  related summarized routes, for the purpose of reducing the size of the Internet routing tables.

IPv6

Whereas IPv4 addresses were 32 bits long, an IPv6 address is 128 bits long. 

IPv6 uses 16-byte hexadecimal number fields separated by colons (:) to represent the 128-bit addressing format. Leading 0s in the address field are optional and can be compressed. A pair of colons (::) represents successive fields of 0. However, the pair of colons is allowed just once in a valid IPv6 address.  

There are three major types of IPv6 addresses:  

   -Unicast: An address for a single interface. A packet that is sent to a unicast address is delivered to the interface identified by that address.   

   -Anycast: An address for a set of interfaces that typically belong to different nodes. A packet sent to an anycast address is delivered to the closest interface, as defined by the routing protocols in use and identified by the anycast address.   

   -Multicast: An address for a set of interfaces (in a given scope) that typically belong to different nodes. A packet sent to a multicast address is delivered to all interfaces identified by the multicast address (in a given scope).    

IPv6 Address Management and Assignment  

There are four ways to configure a host address in IPv6:  

   -Static Configuration: Similar to IPv4, the host address, mask, and gateway address are manually defined.   

   -Stateless Address Autoconfiguration (SLAAC): In this case, the host autonomously configures its own address. Router solicitation messages are sent by booting nodes to request Router advertisements (RA) for configuring the interfaces.

   -Stateful DHCPv6: similar to IPv4 DHCP

   -Stateless DHCP: The host uses SLAAC and also DHCP to get additional parameters such as TFTP Server, WINS, and so on.    

Static address configuration is typically used for router interface configuration but is not likely to be used for hosts in IPv6.

A host can then build its address by appending a host identifier to the /64 prefix received from the router. As a result, Ethernet hosts can autoconfigure themselves by appending their 48-bit link-layer address (MAC address) in an extended universal identifier EUI-64-bit format to the 64 bits of the local link prefix advertised by the router.  

The process for acquiring configuration data for a client in IPv6 is similar to that in IPv4. However, DHCPv6 uses multicast for many of its messages. Initially, the client must first detect the presence of routers on the link using neighbor discovery messages. If a router is found, the client examines the router advertisements to determine whether DHCP should be used.

Stateless DHCPv6 normally combines stateless autoconfiguration for address assignment with DHCPv6 exchange for all other configuration settings. In this case, DHCPv6 is only used for the host to acquire additional parameters, such as a TFTP server, a DNS server, and so on.  

IPv6 Transition Technologies  

Three IPv6 transition technologies: dual stack, tunneling, and translation.  

Dual Stack: It involves configuring devices to be able to run IPv4 and IPv6 simultaneously. Applications choose between using IPv4 or IPv6 based on the response to DNS requests. Because IPv4 addresses are still required, dual stack is not a long-term solution to address exhaustion

Tunneling: Tunnels encapsulate IPv6 traffic within IPv4 packets, and are primarily used for communication between IPv6 (or dual stack) sites or for connection to remote IPv6 networks or hosts over an IPv4 backbone.

Translation: Address Family Translation (AFT) is the process of translating addresses from one address family to another.