Sanu VargheseIT & AI notes

Tech facts

Interesting facts and the numbers behind everyday IT. Good for interviews, quizzes and impressing the rest of the service desk.

Fact of the day

Tech by numbers

4.29 bnIPv4 addresses

2 to the power of 32. Not enough for every device, which is why NAT exists.

340 undecillionIPv6 addresses

2 to the power of 128. Enough for every grain of sand, many times over.

65,535Highest port number

TCP and UDP ports run from 0 to 65,535. DNS is 53, HTTPS is 443, RDP is 3389.

5 minKerberos clock limit

The default maximum time difference before domain sign-ins fail.

8 daysDefault DHCP lease

What a new Windows Server DHCP scope uses unless you change it.

100 mMax copper Ethernet run

The standard limit for a Cat5e or Cat6 channel, patch leads included.

180 daysAD tombstone lifetime

How long deleted AD objects are kept, and why you never restore an old DC snapshot.

13DNS root server names

a to m.root-servers.net, served by well over a thousand machines worldwide.

Did you know?

  1. In 1947, operators of the Harvard Mark II found a moth stuck in a relay and taped it into the logbook as the "first actual case of bug being found".History
  2. Ray Tomlinson sent the first networked email in 1971 and chose the @ sign to separate the user from the computer's name.Email
  3. symbolics.com was the first .com domain ever registered, on 15 March 1985.Internet
  4. The first website, info.cern.ch, was built by Tim Berners-Lee at CERN and opened to the public in 1991. It's still online.Internet
  5. Ethernet was invented at Xerox PARC in 1973 by Bob Metcalfe and David Boggs.Networking
  6. Before DNS was designed in 1983, every computer on the early internet used one shared list of names called HOSTS.TXT.DNS
  7. The hosts file in C:\Windows\System32\drivers\etc is a leftover of that old system, and it's still checked before DNS. Malware loves editing it.Windows
  8. IPv4 has 4,294,967,296 addresses. The last free blocks were handed to the regional registries in February 2011.Networking
  9. IPv6 has 2^128 addresses: about 340 undecillion, or 340 followed by 36 zeros.Networking
  10. Ctrl+Alt+Del was created by IBM engineer David Bradley for the original IBM PC.History
  11. Bluetooth is named after Harald "Bluetooth" Gormsson, a 10th-century Danish king. Its logo joins his initials in runes.Wireless
  12. The first Wi-Fi standard, IEEE 802.11, was published in 1997 and topped out at 2 Mbps.Wireless
  13. Active Directory first shipped with Windows 2000 Server.Windows Server
  14. By default, Kerberos sign-ins fail if a PC's clock is more than 5 minutes out from the domain controller. That's why time sync matters.Windows Server
  15. A new Windows DHCP scope hands out leases of 8 days by default.Windows Server
  16. The 2017 paper "Attention Is All You Need" introduced the Transformer. It's the T in GPT.AI
  17. ChatGPT launched on 30 November 2022 and was estimated to reach 100 million users in about two months.AI
  18. Cisco was founded in 1984 by Leonard Bosack and Sandy Lerner, a married couple who worked at Stanford University.Cisco
  19. Claude Shannon's 1948 paper founded information theory. It was the first published use of the word "bit", which he credited to John Tukey.History
  20. There are only 13 DNS root server names (a to m.root-servers.net), but anycast lets well over a thousand servers worldwide answer for them.DNS
  21. A MAC address is 48 bits long. The first half identifies the manufacturer of the network card.Networking
  22. Copper Ethernet cabling such as Cat5e and Cat6 is specified for runs of up to 100 metres.Networking
  23. Brain, from 1986, is generally considered the first PC virus. It was written by two brothers in Lahore, Pakistan.Security
  24. Junk email is called spam because of a Monty Python sketch in which the word drowns out everything else.Email
  25. Microsoft was founded on 4 April 1975 by Bill Gates and Paul Allen.Microsoft
  26. 127.0.0.1 isn't the only loopback address. The whole 127.0.0.0/8 block, over 16 million addresses, points back at your own machine.Networking
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