Good IP addressing design uses summarizable blocks of addresses that enable route summarization and provide a number of benefits:
- Reduced router workload and routing traffic
- Increased network stability
- Faster convergence
- Significantly simplified troubleshooting
Creating and using summary routes depends on the use of summarizable blocks of addresses. Sequential numbers in an octet may denote a block of IP addresses as summarizable. For sequential numbers to be summarizable, the block must be X numbers in a row, where X is a power of 2, and the first number in the sequence must be a multiple of X. The created sequence will then end one before the next multiple of X in all cases.
Efficiently assigning IP addresses to the network is a critical design decision, impacting the scalability of the network and the routing protocols that can be used. IPv4 addressing has the following characteristics:
- IPv4 addresses are 32 bits in length.
- IPv4 addresses are divided into various classes (e.g., Class A networks accommodate more than 16 million unique IP addresses, Class B networks support more than 65 thousand IP addresses, and Class C networks permit 254 usable IP addresses). Originally, organizations applied for an entire network in one of these classes. Today, however, subnetting allows an ISP to give a customer just a portion of a network’s address space, in an attempt to conserve the depleting pool of IP addresses. Conversely, ISPs can use supernetting (also known as Classless Inter-Domain Routing – CIDR) to aggregate the multiple network address spaces that they have. Aggregating multiple network address spaces into one address reduces the amount of route entries a router must maintain.
- Devices such as PCs can be assigned a static IP address, by hard coding the IP address in the device’s configuration. Alternatively, devices can dynamically obtain an address from a DHCP server, for example.
- Because names are easier to remember than IP addresses are, most publicly accessible Web resources are reachable by their name. However, routers must determine the IP address with which the name is associated to route traffic to that destination. Therefore, a DNS server can perform the translation between domain names and their corresponding IP addresses.
- Some IP addresses are routable through the public Internet, whereas other IP addresses are considered private and are intended for use within an organization. Because these private IP addresses might need to communicate outside the LAN, NAT can translate a private IP address into a public IP address. In fact, multiple private IP addresses can be represented by a single public IP address using NAT. This type of NAT is called Port Address Translation (PAT) because the various communication flows are identified by the port numbers they use to communicate with outside resources.
When beginning to design IP addressing for a network, the following aspects must be determined:
- The number of network locations that need IP addressing
- The number of devices requiring an IP address at each location
- Customer-specific IP addressing requirements (e.g., static IP addressing versus dynamic IP addressing)
- The number of IP addresses that need to be contained in each subnet (e.g., a 48-port switch in a wiring closet might belong to a subnet that supports 64 IP addresses)
A major challenge with IPv4 is the limited number of available addresses. A newer version of IP, specifically IPv6, addresses this concern. An IPv6 address is 128 bits long, compared to the 32-bit length of an IPv4 address.
To make such a large address more readable, an IPv6 address uses hexadecimal numbers and the 128-bit address is divided into eight fields. Each field is separated by a colon, as opposed to the four fields in an IPv4 address, which are each separated by a period. To further reduce the complexity of the IPv6 address, leading 0s in a field are optional and if one or more consecutive fields contain all 0s, those fields can be represented by a double colon (::). A double colon can be used only once in an address; otherwise, it would be impossible to know how many 0s are present between each pair of colons.
Consider some of the benefits offered by IPv6:
- IPv6 dramatically increases the number of available addresses.
- Hosts can have multiple IPv6 addresses, allowing those hosts to multi-home to multiple ISPs.
- Other benefits include enhancements relating to QoS, security, mobility, and multicast technologies.
Unlike IPv4, IPv6 does not use broadcasts. Instead, IPv6 uses the following methods for sending traffic from a source to one or more destinations:
- Unicast (one-to-one): Unicast support in IPv6 allows a single source to send traffic to a single destination, just as unicast functions in IPv4.
- Anycast (one-to-nearest): A group of interfaces belonging to nodes with similar characteristics (e.g., interfaces in replicated FTP servers) can be assigned an anycast address. When a host wants to reach one of those nodes, the host can send traffic to the anycast address and the node belonging to the anycast group that is closest to the sender will respond.
- Multicast (one-to-many): Like IPv4, IPv6 supports multicast addressing, where multiple nodes can join a multicast group. The sender sends traffic to the multicast IP address and all members of the multicast group receive the traffic.
The migration of an IPv4 network to an IPv6 network can take years because of the expenditures of upgrading equipment. Therefore, during the transition, IPv4-speaking devices and IPv6-speaking devices need to coexist on the same network. Consider the following solutions for maintaining both IPv4 and IPv6 devices in the network:
- Dual stack: Some systems (including Cisco routers) can simultaneously run both IPv4 and IPv6, allowing communication to both IPv4 and IPv6 devices.
- Tunneling: To send an IPv6 packet across a network that uses only IPv4, the IPv6 packet can be encapsulated and tunneled through the IPv4 network.
- Translation: A device, such as a Cisco router, could sit between an IPv4 network and an IPv6 network and translate between the two addressing formats.
IPv6 allows the use of static routing and supports specific dynamic routing protocols that are variations of the IPv4 routing protocols modified or redesigned to support IPv6:
- RIPng
- OSPFv3
- EIGRPv6
- IS-IS
- BGP