Capacity and reliability arithmetic for networks, servers and storage, each computed for a worked example and matching the calculators. For IPv4 subnets see the CIDR and Subnet Mask Cheat Sheet.
Formulas
| Measure | Formula | Example |
|---|---|---|
| Transfer time | seconds = file size in bits / usable bits per second |
1 GiB over 100 Mbit/s: 85.9 s; with 10% overhead: 95.4 s |
| Allowed downtime | downtime = period × (1 − availability) |
99.9%: 8.77 hours a year, 43.8 minutes a month; 99.99%: 52.6 minutes a year; 99.999%: 5.26 minutes a year |
| Availability of components in series | A = A₁ × A₂ × … |
Two 99.9% parts that must both work: 99.80% |
| Availability of redundant components | A = 1 − (1 − A₁)(1 − A₂) … |
Two 99.9% parts where either is enough: 99.9999% |
| RAID usable capacity | RAID 1: one drive; RAID 5: (n − 1); RAID 6: (n − 2); RAID 10: n / 2 (times drive size) |
4 TB drives: RAID 1 of 2 = 4 TB; RAID 5 of 4 = 12 TB; RAID 6 of 6 = 16 TB; RAID 10 of 4 = 8 TB |
| IPv6 subnet sizes | addresses = 2^(128 − prefix) |
A /64 holds 18,446,744,073,709,551,616 addresses; a /48 holds 65,536 /64 subnets; a /56 holds 256 |
Try them
The Bandwidth Calculator estimates transfer times, the SLA Uptime Calculator converts uptime percentages to downtime and combines components, and the RAID Calculator shows usable capacity and fault tolerance. For IPv6 use the IPv6 Subnet Calculator. Network speeds are quoted in bits and file sizes in bytes, so divide by 8 when you compare them.
IPv6 addresses and names
- Unique local addresses (ULA) are private IPv6 addresses in
fc00::/7, in practicefdfollowed by a random 40-bit ID:fd12:3456:789a::/48is one. The IPv6 ULA Generator makes a random prefix for a private network. - Random test addresses. The Random IP Address Generator and Random MAC Address Generator make addresses for testing. A MAC with the second-lowest bit of its first byte set, such as
02:00:5e:10:11:22, is locally administered rather than assigned to a vendor. - Internationalised domain names are stored as ASCII:
münchen.debecomesxn--mnchen-3ya.de(Punycode). Decode one with the Punycode to Hostname Converter.
Notes
- Each extra nine is ten times stricter. 99.9% allows about 8.8 hours of downtime a year, 99.99% about 53 minutes.
- Series versus parallel. Chains of components get less reliable (99.80% for two 99.9% parts), while redundancy raises it (99.9999%).
- RAID is not a backup. It survives drive failures, not deletion, corruption or fire.