/19 Subnet: Mask 255.255.224.0

How many /19 networks fit

A /19 holds 32 × /24. Eight /19s make a /16, so you can fit a /19 per AZ and still leave plenty of the VPC unused.

Worked example: 10.1.77.200/19

An EKS node at 10.1.77.200 sits in one of the cluster's /19 private subnets. Which one, and where does it end?

1. The mask is 255.255.224.0. The third octet (224) is the "interesting" one — the first that is not 255.
2. Block size = 256 − 224 = 32. Networks start at multiples of 32 in the third octet: 0, 32, 64, …
3. The third octet of 10.1.77.200 is 77. 77 ÷ 32 = 2 remainder 13, so the block starts at 2 × 32 = 64.
4. Network = 10.1.64.0 (octets after the third become 0). Broadcast = 64 + 32 − 1 = 95, giving 10.1.95.255 (octets after it become 255).
5. Usable hosts: 10.1.64.1 to 10.1.95.254, which is 2¹³ − 2 = 8,190 addresses.

Address  00001010.00000001.010|01101.11001000
Mask     11111111.11111111.111|00000.00000000
Network  00001010.00000001.010|00000.00000000

Where /19 is used in real networks

• EKS and AKS pod subnets

  With the AWS VPC CNI or Azure CNI, every pod uses an address from the subnet. A /19 per AZ gives about 8,000 IPs per zone, which avoids the classic "insufficient IP addresses" errors when nodes scale out. Size it with the EKS max pods calculator.

• Eight equal blocks per VPC

  Cutting a /16 into eight /19s is a tidy plan: three private, three public and two spare, for example.

Common /19 mistakes

• Writing 10.1.48.0/19. The third octet must be a multiple of 32 (0, 32, 64, 96 …), so it becomes 10.1.32.0/19.
• Using a /24 for EKS private subnets. With 30 pods per node, a /24 runs out after about eight nodes. A /19 is a much safer starting point.

/19 FAQ

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