AppHelp guide
Subnets, CIDR Ranges, DNS Records, Ports, and MAC Addresses
A practical network operations guide: size subnets, turn IP ranges into CIDR lists, check DNS records, look up ports, and identify devices by MAC address.
Quick answer
Use the subnet calculator to size and split networks, the IP range to CIDR converter to build compact allowlists, DNS lookup to confirm what public resolvers return, the port reference to decide what a firewall should allow, and MAC lookup to identify devices in ARP or DHCP tables.
Size the network before you assign addresses
A prefix length fixes how many addresses a subnet holds. A /24 has 254 usable IPv4 hosts, a /26 has 62, and /31 point-to-point links use both addresses. Split a larger block into equal subnets by how many you need or by hosts per subnet.
- Leave room for growth: pick the next larger subnet when you are close to the limit
- Use wildcard masks (inverse netmasks) for Cisco ACLs and OSPF
- IPv6 LANs are normally /64; plan by number of /64s instead of hosts
Keep firewall and allowlist entries compact
Vendors often publish start–end ranges, while firewalls and WAF rules want CIDR. Convert ranges to the minimal CIDR set and merge overlapping or adjacent blocks so lists stay short and auditable.
Check DNS at public resolvers after every change
Office and ISP resolvers cache answers until the TTL expires. Query Cloudflare and Google directly to see what the internet sees, and check MX, SPF and DMARC together before sending mail from a new domain.
Only expose the ports a service needs
Web traffic needs 80 and 443. Databases, Redis, Docker API, SMB and RDP should stay behind a VPN or an IP allowlist. Look up unknown ports from a scan before deciding whether to close them.
Identify devices by MAC address
The first three bytes of a MAC address (the OUI) identify the manufacturer. Addresses whose second hex digit is 2, 6, A or E are locally administered, typically randomised by phones for privacy, so they have no vendor.
Frequently asked questions
How do I choose a subnet size?
Count the hosts you need now and in the near future, add network and broadcast addresses for IPv4, and pick the smallest prefix that fits. The subnet calculator's hosts-per-subnet mode does this for you.
Why does DNS show different answers in different places?
Each resolver caches records for their TTL. After a change, older answers can persist until the TTL expires.
Can I tell a device's manufacturer from its MAC address?
Yes, for globally unique addresses. Look up the OUI prefix in the IEEE registry. Randomised (locally administered) addresses have no vendor.