Code-Memo

Subnetting & CIDR

Why subnetting exists

• Isolation: reduce broadcast domains, segment environments (prod/dev), security boundaries.
• Scalability: organize address space and route aggregation.
• Efficiency: allocate addresses sized to needs instead of wasting large blocks.

Network vs host bits

• In a.b.c.d/N, the first N bits identify the network; remaining bits identify the host.
• Hosts share a subnet if their network bits match under the mask.

Subnet masks explained

• A subnet mask is the IPv4 way to represent CIDR, e.g.:
  • /24 → 255.255.255.0
  • /16 → 255.255.0.0
• Mask bits set to 1 indicate the network portion.

VPC subnet design in cloud

• Start with a VPC CIDR (e.g., 10.0.0.0/16) and carve subnets per AZ/purpose.
• Typical split: public subnets (route to Internet Gateway) vs private subnets (no direct inbound Internet).
• Leave room for growth; avoid overlaps with on‑prem if you plan VPN/Direct Connect/peering.

Practical subnet calculations

• IPv4 address count in a /N block: (2^{(32-N)}).
• Example: /26 → (2^{6}=64) addresses (usable may be fewer depending on platform reservations).
• Sanity check by writing the block boundaries (increments) in the relevant octet.