Network and homelab: how the math is checked

How the math works

• A prefix of /p leaves 32 - p host bits, which is 2 to that power addresses in the block.
• Usable hosts are the block size minus 2 (the network and broadcast addresses). A /31 is a special case for point-to-point links and uses both addresses. A /32 is a single host.
• VLSM picks the smallest power of two with room for the hosts, then aligns each block to its own size.

Leave room to grow. If a VLAN needs 100 hosts today, plan for the next size up.

Worked example

192.168.1.130/26 is in the network 192.168.1.128, with broadcast 192.168.1.191 and mask 255.255.255.192. The usable range is .129 to .190, which is 62 hosts. For VLSM, split 192.168.0.0/24 for 100, 50, 25 and 10 hosts. You get 192.168.0.0/25, 192.168.0.128/26, 192.168.0.192/27 and 192.168.0.224/28, which uses 240 of the 256 addresses.

How the math works

• Draw for each line is the count times the watts per device.
• Total draw is the sum. Headroom is the budget minus the total, and utilization is total divided by budget.

Use the figure from the device's data sheet, measured at the switch port. The standards set per-port ceilings at the switch: about 15.4 W for 802.3af, 30 W for 802.3at and 60 W or 90 W for 802.3bt. Many devices draw less than the ceiling, but cameras with heaters and some access points can spike, so add margin.

Worked example

A switch has a 370 W budget. You plan 10 phones at 7 W, 4 access points at 15.4 W and 6 cameras at 12 W. That is 70 + 61.6 + 72 = 203.6 W, or about 55 percent of the budget, with 166.4 W left. Adding eight 30 W devices later would take you to 443.6 W, which is over budget.

How the math works

• Battery energy in watt-hours is volts times amp-hours times the number of batteries.
• Usable energy is that figure times the depth of discharge you allow and times the inverter efficiency.
• Runtime in hours is usable watt-hours divided by the load in watts.

Real runtime is shorter when batteries are old, cold or discharged fast, because capacity falls as the draw rises. Lead-acid batteries last longer if you do not drain them deeply, so many people plan for around half or less. Measure your real load with a plug-in meter instead of adding up label ratings.

Worked example

Two 12 V, 9 Ah batteries hold 12 x 9 x 2 = 216 Wh. With 80 percent usable depth and 85 percent efficiency, usable energy is 146.9 Wh. At a 150 W load that is 0.98 hours, about 59 minutes. If the load is only 75 W, the estimate doubles to nearly two hours.

How the math works

• Energy per day in kWh is count times watts times hours, divided by 1,000.
• The overhead percentage, if any, is added on top.
• Yearly energy is daily energy times 365. Cost is energy times your price per kWh. The monthly figure is the yearly cost divided by 12.

Use a plug-in watt meter for real numbers. The power supply rating on the label is a ceiling, not what the machine usually draws, and idle draw is often far lower than load.

Worked example

One server at 100 W running 24 hours uses 2.4 kWh a day, which is 876 kWh a year. At 0.15 per kWh that costs 131.40 a year, or 10.95 a month. Two 50 W devices running 10 hours a day with a 10 percent cooling overhead use 1.1 kWh a day.

How the math works

• Access capacity is the sum of port count times speed for the access ports.
• Uplink capacity is the sum of count times speed for the uplinks.
• The ratio is access capacity divided by uplink capacity.

A lower ratio is safer. What is acceptable depends on how busy your users are and what they do. Backups, video and storage traffic need more headroom than office browsing. Treat any ratio as a rule of thumb, and watch real utilization on the uplink over time.

Worked example

A switch has 48 ports at 1,000 Mbps, for 48,000 Mbps of access capacity. It has two uplinks at 10,000 Mbps each, for 20,000 Mbps. The ratio is 48,000 / 20,000 = 2.4 to 1. If you drop to a single uplink, the ratio rises to 4.8 to 1.