When to use: Plan VLAN segmentation strategy at project start. Enter each logical segment (users, servers, VoIP, IoT, guest) with host count to get the minimum subnet that fits without wasteful over-allocation. Helps align VLAN IDs, network names, and /prefix assignments before configuring managed switches and router ACLs.
| VLAN ID | Name | Hosts | Purpose | Recommended Subnet | Usable IPs | Waste | |
|---|---|---|---|---|---|---|---|
| /24 | 254 | 54 | |||||
| /26 | 62 | 12 | |||||
| /25 | 126 | 26 | |||||
| /27 | 30 | 0 | |||||
| /25 | 126 | 46 |
This tool generates recommended subnet prefix lengths for each VLAN segment based on host count, showing usable IP addresses and address waste per segment. Network engineers use it at project initiation to structure their IP addressing plan and align VLAN IDs with subnet assignments before configuring managed switches and routers.
The minimum subnet for a given host count is determined by finding the smallest prefix length where 2^(32−prefix) − 2 ≥ hosts_required. For 200 hosts, a /24 (254 usable) is sufficient; a /25 (126 usable) is too small. The /24 subnet size is not mandatory — CIDR allows any prefix length from /1 to /30. Matching the prefix tightly to the host count minimizes address waste, which is important in large networks with hundreds of VLANs.
Each VLAN is a separate broadcast domain. All hosts in a VLAN share the same subnet and communicate at Layer 2 without routing. Traffic between VLANs requires a Layer 3 device (router or Layer 3 switch SVI). Larger subnets increase broadcast traffic — keeping VLANs to /24 or smaller is a standard best practice.
IEEE 802.1Q defines VLAN tagging (4-byte tag with 12-bit VLAN ID, supporting VLANs 1–4094 with 1 and 4094 reserved). RFC 1918 defines private IPv4 address space for internal VLAN subnets. RFC 4632 defines CIDR notation. DHCP scope assignments per RFC 2131 must match subnet boundaries. Best practice guidelines from Cisco's SRND recommend separating voice, data, server, management, IoT, and guest into dedicated VLANs for security segmentation and QoS policy enforcement. NIST SP 800-125B covers network segmentation for virtualized environments that follow similar VLAN design principles.
Avoid placing more than 250 hosts in a single VLAN broadcast domain — large broadcast domains increase ARP storm risk and reduce security segmentation. Use a /22 or /23 only for guest Wi-Fi where many personal devices connect simultaneously and segregation is less critical. The management VLAN (for switch, AP, UPS, and PDU management interfaces) should be a small subnet (/27 or /28) with strict ACL controls preventing user-to-management access. IoT and OT devices should always be isolated in a dedicated VLAN with firewall enforcement between IoT and corporate segments due to their poor security posture.
Enter each logical network segment as a VLAN with a unique VLAN ID (1–4094), a descriptive name, and the expected host count. Add or remove VLANs using the table controls. The recommended subnet column shows the minimum prefix length for each segment. Review the waste column — high waste (>50%) indicates the next smaller prefix should be used if future growth is limited. Use the VLAN ID and subnet prefix in your switch configuration (interface VLAN, ip address, ip helper-address) and DHCP scope definitions.
IEEE 802.1Q supports VLAN IDs 1–4094 (12 bits), with 1 reserved as default and 4094 reserved. Most enterprise switches support the full range. However, VTP (VLAN Trunking Protocol) in versions 1 and 2 limits propagation to 1005 VLANs; use VTP version 3 or manual VLAN configuration for larger deployments.
No. Best practice is a separate voice VLAN for IP phones with DSCP EF marking applied at the access switch. Separating voice from data allows QoS policies to apply per-VLAN, simplifies troubleshooting voice quality issues, and prevents data traffic from impacting voice quality during congestion.
The native VLAN is the VLAN whose traffic is sent untagged on 802.1Q trunk links. Both ends of a trunk must agree on the native VLAN or mismatched frames will be forwarded to the wrong VLAN. Best practice is to set the native VLAN to an unused VLAN ID (not VLAN 1) to prevent VLAN hopping attacks.
Plan for at least 30–50% unused addresses at deployment to allow organic growth without subnet re-addressing. For a /24 (254 usable), deploying no more than 175 hosts initially leaves adequate headroom. Subnet re-addressing is disruptive and requires reconfiguring DHCP, DNS, firewall ACLs, and switch configurations simultaneously.
A VLAN is a Layer 2 logical grouping of switch ports (defined by IEEE 802.1Q). A subnet is a Layer 3 IP address range (defined by CIDR). In practice, each VLAN is assigned exactly one subnet and a router or SVI provides the default gateway for that subnet. The mapping is one-to-one in most enterprise designs.
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