Reference
Field notes
The rules every IT Field Kit answer comes from, so you can check them.
Subnets
A prefix of /n means the first n bits are the network. A block holds 232-n addresses; the first is the network address and the last the broadcast, so a host can use 232-n - 2. Two exceptions: a /31 is a point-to-point link where both addresses are usable (RFC 3021), and a /32 is a single address.
Private ranges (RFC 1918): 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. 100.64.0.0/10 is shared address space for carrier-grade NAT (RFC 6598). 169.254.0.0/16 is link-local: a Windows PC that shows 169.254.x.x got no DHCP answer.
VLAN and subnet planning
Each network gets the smallest block whose usable count covers the devices you list. Blocks are placed biggest first, which keeps every block on its own boundary and leaves the free space in as few pieces as possible. Leave room to grow: a network planned at 90% full will be full soon.
PoE power
A switch sets aside power for each port by the device's IEEE class. These are the watts at the switch port (the device itself may draw a little less, the rest is lost in the cable):
| Class | Switch sets aside | Device may draw | Port needed |
|---|---|---|---|
| 0 | 15.4 W | 13 W | 802.3af (PoE) port |
| 1 | 4.0 W | 3.84 W | 802.3af (PoE) port |
| 2 | 7.0 W | 6.49 W | 802.3af (PoE) port |
| 3 | 15.4 W | 13 W | 802.3af (PoE) port |
| 4 | 30.0 W | 25.5 W | 802.3at (PoE+) port |
| 5 | 45.0 W | 40 W | 802.3bt Type 3 port |
| 6 | 60.0 W | 51 W | 802.3bt Type 3 port |
| 7 | 75.0 W | 62 W | 802.3bt Type 4 port |
| 8 | 90.0 W | 71.3 W | 802.3bt Type 4 port |
Source: IEEE 802.3af (classes 0-3), 802.3at (class 4, "PoE+") and 802.3bt (classes 5-8). Many switches can also budget by what a port actually draws or by LLDP; IT Field Kit shows the class-based worst case, which is what most switches use by default. Check your switch's PoE mode and its total budget on the datasheet.
Transfer time
Time = data in bits / (link speed x efficiency). 1 GB = 1,000,000,000 bytes (1 GiB = 1,073,741,824). A link rarely carries data at its full rated speed once protocol overhead, disks and small files are counted; 70-90% is common on a wired LAN, much less over Wi-Fi or a VPN. Measure one real copy and use that percentage.
RAID
- RAID 0: all disks' space, no disk may fail.
- RAID 1: one disk's space, every other disk may fail.
- RAID 5: one disk's space goes to parity, one disk may fail (needs 3+).
- RAID 6: two disks' space goes to parity, two may fail (needs 4+).
- RAID 10: half the space, one failure for sure, more only if they hit different mirror pairs (even count, 4+).
Drive makers count TB in decimal; operating systems often show TiB, which is about 9% smaller. RAID is not a backup: it does nothing against deletion, ransomware or a failed controller.