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Difference Between VLSM and CIDR

 VLSM (Variable Length Subnet Mask) and CIDR (Classless Inter-Domain Routing) are both techniques for efficient IP address allocation, but they serve different purposes:

  1. VLSM (Variable Length Subnet Mask):

    • VLSM allows different subnets within the same network to use different subnet masks, making it possible to allocate IP addresses more efficiently based on need (i.e., smaller subnets for smaller networks, larger subnets for larger ones).
    • Used mainly within internal networks (intra-domain) to maximize the usage of available IP space.
    • Requires routers that support classless routing protocols (e.g., OSPF, EIGRP, or RIPv2).

  2. CIDR (Classless Inter-Domain Routing):

    • CIDR is a method of assigning IP addresses without adhering to the traditional class-based system (A, B, C), allowing for more flexible and hierarchical IP address allocation.
    • CIDR is primarily used for routing between networks (inter-domain), particularly on the Internet, to reduce routing table sizes and prevent IP exhaustion.
    • Expressed as IP/Prefix-Length (e.g., 192.168.0.0/16), CIDR allows for supernetting or the aggregation of IP addresses.

Key Difference:
VLSM is focused on subnetting within a network using different masks, while CIDR is about aggregating IP addresses for routing purposes, eliminating the need for class-based addressing altogether.

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