Understanding CIDR, Subnetting, and Supernetting in IP Addressing

Drawbacks of Classful Addressing

Subnets have a fixed number of host addresses, leading to many unused/wasted IP addresses.

Solutions for IP Address Shortages

Short-term: Classless addressing
Long-term: IPv6

CIDR (Classless Inter-Domain Routing)

CIDR uses a hierarchical addressing scheme based on VLSM (Variable Length Subnet Masking).

Advantages of CIDR

• Replaces classful addressing with a more flexible and less wasteful classless scheme.
• Enhances route aggregation.

How VLSM Works

VLSM further subnets existing subnets by taking bits from the host ID, creating a multi-level hierarchy with "sub-subnets."

Supernetting

Supernetting combines continuous network addresses into a new address defined by the subnet mask. Multiple networks are combined into a larger network called a supernetwork or supernet.

Primarily used in route summarization, supernetting combines routes to multiple networks with similar prefixes into a single routing entry. This entry points to a supernet encompassing all the networks, reducing routing table size and routing update sizes.

Supernetting benefits include more efficient routing, reduced CPU cycles for routing table updates, and lower router memory requirements.

Use of a 30-Bit Mask

A 30-bit mask is used for point-to-point serial links, providing two host addresses.

Commonality of Summarized Addresses

Summarized addresses share high-order bits.

Route Flapping and its Impact on Route Aggregation

Route flapping occurs when a router interface rapidly alternates between up and down states. Route aggregation can help insulate routers from this problem, as the lost network might already be included in the advertised supernet address.

NAT (Network Address Translation)

NAT swaps one IP address for another in the IP packet header.

Disabling Automatic Summarization

The Cisco command no auto-summary disables automatic summarization.

Explanation of NAT

A private host transmits a packet to an internet destination. The NAT device (router) receives the packet and replaces the inside IP address in the header with a globally unique IP address. When the outside host sends a response, the router receives it, checks the NAT correspondence table, and replaces the destination address with the original internal source address.

Types of NAT

• Dynamic NAT: Uses a NAT pool (a group of public IP addresses).
• Static NAT: Provides a one-to-one mapping of addresses.

Many-to-One NAT (PAT)

PAT (Port Address Translation) maps multiple inside addresses to the same public address. The NAT router tracks different conversations by mapping TCP and UDP ports.

Unnumbered Serial Interface

An unnumbered serial interface does not require its own IP address.

IP Helper Addresses

IP helper addresses enable a router to accept a broadcast request for a UDP service and forward it as a unicast to a specific IP address.