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IP CIDR

·752 words·4 mins
lab1918
Author
lab1918

IPv4 addressing can be understood in terms of “classful” and “classless” systems. These concepts are fundamental to how IP addresses are allocated and interpreted.

Classful Addressing
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Classful addressing was the original framework used in the early stages of the Internet. In this system, the IP address space was divided into fixed-size classes, each identified by the leading bits in the address:

  1. Class A: Addresses began with a 0 bit. The first 8 bits (the first octet) identified the network, and the remaining 24 bits were for host addresses. This allowed for a small number of networks, but each with a large number of hosts (e.g., 10.0.0.0 to 10.255.255.255).

  2. Class B: Addresses began with 10. The first 16 bits were for the network, and the remaining 16 bits for hosts. This allowed for more networks than Class A but with fewer hosts per network.

  3. Class C: Addresses began with 110. The network was identified by the first 24 bits, leaving only 8 bits for host addresses. This allowed for many networks but with a limited number of hosts on each.

  4. Class D: Used for multicast, these addresses began with 1110.

  5. Class E: Beginning with 1111, these were reserved for experimental purposes.

Classful addressing, used in the early development of the Internet, divided the IPv4 address space into five classes: A, B, C, D, and E. Each class had a specific range of addresses, defined by the first few bits in the IP address and the default subnet mask. Here are the ranges for each class:

Class A
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  • Range: 1.0.0.0 to 126.0.0.0
  • First Bit: 0
  • Default Subnet Mask: 255.0.0.0 (/8)
  • Characteristics: Designed for very large networks, like multinational corporations. Each Class A network could support 16,777,214 hosts.

Class B
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  • Range: 128.0.0.0 to 191.255.0.0
  • First Bits: 10
  • Default Subnet Mask: 255.255.0.0 (/16)
  • Characteristics: Used for medium-sized networks, such as universities or large businesses. Each Class B network could support 65,534 hosts.

Class C
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  • Range: 192.0.0.0 to 223.255.255.0
  • First Bits: 110
  • Default Subnet Mask: 255.255.255.0 (/24)
  • Characteristics: Intended for small-sized networks, like small businesses. Each Class C network could support 254 hosts.

Class D (Multicast)
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  • Range: 224.0.0.0 to 239.255.255.255
  • First Bits: 1110
  • Characteristics: Reserved for multicast groups, not for standard host addresses.

Class E (Reserved)
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  • Range: 240.0.0.0 to 255.255.255.255
  • First Bits: 1111
  • Characteristics: Reserved for future or experimental purposes and not for public use.

Important Notes
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  • Network and Broadcast Addresses: In each network, the first address is reserved as the network address, and the last address is reserved as the broadcast address. These should not be assigned to individual hosts.
  • Classful Addressing Limitations: This system was eventually deemed inefficient due to its rigid structure, leading to the development of Classless Inter-Domain Routing (CIDR).
  • CIDR: With CIDR, the classful system was replaced by a more flexible system that allowed for variable-length subnet masking (VLSM), significantly improving the efficiency of IP address allocation.

Understanding classful addressing is more of historical interest and foundational knowledge in networking, as the modern Internet relies on CIDR for IP address allocation and routing.

Classless Addressing (CIDR)
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Classless Inter-Domain Routing (CIDR) was introduced to improve the efficiency and flexibility of IP address allocation. It replaced the classful system in the mid-1990s:

  1. Variable Length Subnet Masking (VLSM): CIDR allows for subnet masks of variable lengths, which means an IP address block can be divided into smaller or larger subnets as needed. This is expressed in a format like 192.168.0.0/24, where /24 indicates that the first 24 bits are the network part.

  2. Efficient Utilization: CIDR provides more efficient use of the IP address space. It addresses the limitations of the classful system, where many IP addresses were wasted due to the rigid boundaries of Class A, B, and C networks.

  3. Route Aggregation: CIDR also allows for route aggregation (or route summarization), which reduces the size of routing tables and improves the efficiency of IP routing.

Key Differences
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  • Flexibility: Classless addressing is more flexible than classful addressing, allowing for a more precise allocation of IP addresses based on the actual need, rather than a fixed class structure.

  • Efficiency: CIDR leads to more efficient use of the IP address space, reducing waste.

  • Routing: CIDR simplifies routing because it can aggregate several smaller networks into a single routing table entry.

In summary, the shift from classful to classless addressing was a critical development in the management of IP address space. It addressed the inefficiencies of the classful system and has been key in delaying the exhaustion of IPv4 addresses.