5) IP Addressing

IPv4 (Internet Protocol version $4$ ) is the fourth version of the Internet Protocol.
It utilize a $32\text{-bit}$ address scheme to provide approximately $4.3 \times 10^9$ (or $4.3$ billion) unique addresses.
Due to the exponential growth of the internet, IPv4 has largely exhausted its available address space.
IPv4 was established in the early $1980\text{s}$ and remains a fundamental system for addressing devices on a network.

Public IP Addresses
- These addresses are globally unique and designated for use on the public Internet.
- Public addresses ensure that each device can be uniquely identified and communicated with by any other device globally.
- Class A Range: $1.0.0.0 - 126.255.255.255$
- Class B Range: $128.0.0.0 - 191.255.255.255$
- Class C Range: $192.0.0.0 - 223.255.255.255$
Private IP Addresses
- Private IP addresses are designated for use within Local Area Networks (LANs).
- They are not routable on the public internet.
- Their primary function is to allow multiple devices within a private network to communicate with one another and with the internet through a translating device (Network Address Translation) without requiring a unique public IP for every single device.
- Class A Range: $10.0.0.0 - 10.255.255.255$
- Class B Range: $172.16.0.0 - 172.31.255.255$
- Class C Range: $192.168.0.0 - 192.168.255.255$

Multicast Address
- Range: $224.0.0.0 - 224.255.255.255$
- Provides service-based group messaging.
- Utilized for one-to-many communications.
Broadcast Address
- Layer 3 Broadcast Address: $255.255.255.255$
- Provides communication to all hosts at the network layer.
- Layer 2 Broadcast Address: $FF:FF:FF:FF:FF:FF$
- Provides communication to all hosts at the datalink layer.
Loopback Address
- Range: $127.0.0.0 - 127.255.255.255$
- Allows a host to communicate with itself.
- Primary use is for testing a network interface.

APIPA is a feature of Windows operating systems.
It automatically assigns a unique IP address when a host fails to obtain an IP address from a DHCP (Dynamic Host Configuration Protocol) server.
Address Range: $169.254.0.0 - 169.254.255.255$ (specifically $169.254.0.1$ to $169.254.255.254$ ).
APIPA allows for automatic, ad hoc network communication within a single subnet when a DHCP server is unavailable.
This mechanism ensures local communication in the absence of manual or DHCP-based configuration, though it does not provide internet access.

IPv4 Subnetting
- Subnetting is the practice of dividing a network into two or more smaller network segments, or subnets.
- It is used to improve efficiency, security, and management of IP address allocations.
- Segmentation is based on requirements for a specific number of hosts or to isolate network traffic to enhance performance.
- It allows for more efficient use of an organization's allocated address space within a physical infrastructure.
Classful Networking
- An early method of IP allocation that divides address space into fixed-length groups (Classes A, B, C, D, and E).
- Each class has a default subnet mask and a predetermined number of networks and hosts.
- While it simplified routing, the rigid structure was inefficient and led to address waste.
Classless Inter-Domain Routing (CIDR) and VLSM
- VLSM (Variable-Length Subnet Mask): Allows subnets to have different sizes based on specific needs, reducing the waste of IP addresses.
- CIDR Notation: A method for specifying IP addresses and their associated routing prefix (e.g., $192.168.1.0/24$ ).
- The slash ("/") followed by a number indicates how many bits are dedicated to the network portion of the address.
- CIDR replaced classful networking to allow for more flexible and efficient IP address allocation across the internet.

IPv6 (Internet Protocol version $6$ ) is the standard designated to replace IPv4.
It employs $128\text{-bit}$ addresses, resulting in a vastly expanded address space of approximately $3.4 \times 10^{38}$ addresses.
Enhanced Security: IPSec (Internet Protocol Security) is built-in as a standard feature, leading to improved security practices.
Improved Performance: IPv6 eliminates broadcast traffic and replaces it with Anycast.
Efficient Communications: IPv6 is multicast-based, making it less wasteful when communicating with multiple devices.
Simplified Configuration and Management:
- Designed for "plug and play" local communication via Link-Local addresses.
- Supports autoconfiguration, where a host can use its MAC address as part of its IPv6 address.

Address Breakdown:
- Total Length: $128\text{ bits}$ .
- Prefix ( $64\text{ bits}$ ): Identifies an anycast network. It consists of a $48\text{-bit}$ global routing prefix and a $16\text{-bit}$ Subnet-ID.
- Interface-ID ( $64\text{ bits}$ ): Identifies a specific host.
- Example Address: $2001:0db8:3c4d:0012:0000:0000:1234:56ab$
Shortened Expression Rules:
- Drop Leading Zeros: Leading zeros in any field can be removed (e.g., converting $0db8$ to $db8$ and $0012$ to $12$ ).
- Zero Compression: A group of four zeros can be noted as a single zero ( $0$ ).
- Double Colon ("::"): Contiguous fields containing only zeros can be replaced by a double colon. This rule can only be applied once in an address to avoid ambiguity.
- Final Shortened Example: $2001:db8:3c4d:12::1234:56ab$

Multicast Address
- Range: $FF00/8$
- Provides service-based group messaging and one-to-many communications.
Loopback Address
- Address: $::1$
- Allows a host to communicate with itself; used for testing the network interface.
Anycast Address
- Range: N/A (determined by structure).
- Identifies an anycast network.
- Used for one-to-nearest communications.
- Replaces the broadcast function found in IPv4.
Unspecified Address
- Address: $::$
- Identified as the default route within a routing table.
- Identified as "any" IP address in filter lists.