/19 Subnet Mask: 255.255.224.0

/19 quick reference

Worked example: 172.16.32.0/19

The third mask octet is 224; 256 - 224 = 32. Therefore the aligned /19 networks inside 172.16.0.0/16 begin at third-octet values 0, 32, 64, 96, 128, 160, 192 and 224.

An address such as 172.16.50.25/19 belongs to this same subnet because 50 falls inside the aligned 32–63 block.

Count and verify the addresses

Thirteen host bits give 2^13 = 8,192 total addresses. Under traditional IPv4 subnetting, subtract the all-zero host value (network) and all-one host value (broadcast), leaving 8,190 usable addresses. The block covers 32 consecutive /24-sized ranges.

The inverse mask is 0.0.31.255. If used in a Cisco ACL or routing statement, the address must be aligned: 172.16.32.0 0.0.31.255 describes the example range. Starting it at 172.16.50.0 would not create a new /19 boundary.

Design use: aggregation rather than one flat LAN

A /19 is often more useful as an allocation or route summary than as one broadcast domain. A campus, data centre or cloud environment can divide it into smaller VLANs/subnets while advertising a single aligned summary toward the core. That reduces routing entries without placing thousands of endpoints in one Layer-2 segment.

Before summarising, confirm that every more-specific subnet inside the /19 follows the same forwarding path. A summary for space that is partly located elsewhere can create a black hole. Address capacity also is not a reason to ignore segmentation, DHCP design, gateway redundancy, security zones or growth planning.

Frequently asked questions

Primary sources

• RFC 1878: Variable Length Subnet Table for IPv4
• Cisco: understand host and subnet quantities
• Cisco IOS: wildcard masks in access lists

Port registrations describe assigned names, not a guarantee about the process actually listening on a device. Confirm the local process and configuration before changing a firewall.

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