GCP Subnet Calculator
Size Google Cloud VPC subnets with the 4 reserved addresses (first two, last two) already subtracted from usable-host counts.
IPv4 Subnet Calculator
Results
Network Map
Summary
GCP Reserved IPs
Google Cloud reserves 4 IP addresses in every subnet: the first two and the last two.
Advanced: View Binary Representation
IP addresses are stored as binary numbers (1s and 0s) in computers. This is how your network looks in binary:
Advanced Features
🔍 Compare Two Subnets
Compare two subnets to see if they overlap, contain each other, or are completely separate.
Comparison Results
The subnets are completely separate - they do not share any IP addresses.
🧮 VLSM Calculator (Variable Length Subnet Masking)
Create multiple subnets of different sizes from a single network. VLSM optimizes IP address utilization by allocating exactly the right subnet size for each requirement.
VLSM Benefits:
- • Reduces IP address waste by up to 80%
- • Automatically calculates optimal subnet sizes
- • Perfect for complex network designs
- • Supports up to 100 different subnet requirements
Need to Validate IP Addresses?
Before configuring your subnets, ensure your IP addresses are properly formatted with our comprehensive IP validator. Supports both IPv4 and IPv6 with detailed analysis and error detection.
Validate IP AddressesAlso see: general subnet calculator · AWS subnet calculator · Azure subnet calculator
GCP reserves 4 IPs per subnet — and they're not where you expect
Google Cloud takes fewer addresses than AWS or Azure, but it takes them from both ends
of the range. In a 10.128.0.0/24 subnet:
10.128.0.0Network address — the first address of the primary range.10.128.0.1Default gateway — the second address, always the subnet's gateway. VMs learn it via DHCP; you never configure a gateway VM.10.128.0.254Second-to-last address — reserved by Google. This is the one that surprises people used to AWS/Azure, where the reservations all sit at the bottom of the range.10.128.0.255Broadcast address — the last address of the range.
A /24 therefore yields 252 usable IPs and the minimum subnet, a /29, yields 4.
When you script address assignments, remember the usable block ends at
.253, not
.254.
Regional subnets and in-place expansion
GCP subnets are regional, not zonal. One subnet spans every zone in its region, so instances in us-central1-a and us-central1-f can share a single range. If you're arriving from AWS, this collapses the usual subnet-per-AZ sprawl: one subnet per region per tier is often enough, which means fewer, larger ranges in your IP plan.
Undersized anyway? GCP is the only major cloud where that's routinely fixable in place:
gcloud compute networks subnets expand-ip-range
grows a subnet's primary range without touching running VMs. Expansion is one-way — you can go
from /24 to /20 but never back — and the enlarged range must not collide with other subnets,
peered VPCs, or routes. Secondary ranges (used for GKE pods and services as alias IPs) follow
their own rules and are planned separately from the primary range this calculator sizes.
Auto-mode VPCs and the 10.128.0.0/9 trap
A new project's default network is an auto-mode VPC: Google pre-creates one /20
subnet per region, all drawn from 10.128.0.0/9
(10.128.0.0 through 10.255.255.255). That entire half of 10/8 is exactly where many corporate
networks already live, so the first VPN or Interconnect back to on-prem — or peering with another
auto-mode VPC — dies on overlapping routes. Because claimed auto-mode ranges can't be changed,
the fix is preventive: create custom-mode VPCs, allocate subnet ranges from your
org-wide IPAM plan, and use this calculator to verify each range's usable capacity before you
commit it. Existing auto-mode networks can be converted to custom mode (irreversibly), which at
least stops new regions from claiming more of 10.128.0.0/9.
GCP Subnet FAQ
How many usable IPs are in a /24 GCP subnet?
252. Google Cloud reserves 4 addresses per subnet — the network address (.0), the default gateway (.1), the second-to-last address (.254), and the broadcast address (.255) — leaving 252 assignable IPs in a /24. That's one more than AWS or Azure, which each reserve 5.
Which IP addresses does Google Cloud reserve in a subnet?
Four per primary IPv4 range: the first address (network), the second address (subnet default gateway), the second-to-last address (reserved by Google for potential future use), and the last address (broadcast). Everything in between is assignable to VMs, internal load balancers, and other resources.
Can I expand a GCP subnet without downtime?
Yes. Run 'gcloud compute networks subnets expand-ip-range SUBNET_NAME --region=REGION --prefix-length=NEW_PREFIX' to grow a subnet's primary range in place — existing VMs keep their IPs and stay running. You can only expand (to a smaller prefix number), never shrink, and the new range must not overlap other subnets or peered networks.
Should I use auto mode or custom mode VPC networks in GCP?
Custom mode for anything serious. Auto mode carves one /20 per region out of 10.128.0.0/9, which very commonly overlaps on-prem 10.x address space and blocks VPC peering or VPN connectivity later. You can convert an auto-mode network to custom mode (one-way), but you cannot change the ranges it already claimed, so starting in custom mode is cleaner.
More Network Tools
Free tools for network engineers — no signup, no rate-limit walls.