Archive for the ‘ General Info ’ Category

Creating a DKIM record in DNS

Adding a DKIM record to your DNS is quite simple. Add two txt records like

_domainkey.domain.com          IN TXT o=!;r=postmaster@domain.com
selector._domainkey.domain.com IN TXT v=DKIM1;k=rsa;p=<public key>

Which means:

  • o=~ the server signs some mail
  • o=- all mail is signed, but unsigned mail should be accepted
  • o=! all mail is signed, do not accept unsigned mail
  • t=y I’m still testing
  • v=DKIM1 we use DKIM version 1
  • k=rsa it is a RSA key
  • r=<x@xx> report problems to this email address
  • p=<public key> this is my public key

Subnet Masks

A subnetwork, or subnet, is a logically visible, distinctly addressed part of a single Internet Protocol network. The process of subnetting is the division of a computer network into groups of computers that have a common, designated IP address routing prefix.

/ binary hex decimal
/1 10000000.00000000.00000000.00000000 0×80000000 128.0.0.0
/2 11000000.00000000.00000000.00000000 0xc0000000 192.0.0.0
/3 11100000.00000000.00000000.00000000 0xe0000000 224.0.0.0
/4 11110000.00000000.00000000.00000000 0xf0000000 240.0.0.0
/5 11111000.00000000.00000000.00000000 0xf8000000 248.0.0.0
/6 11111100.00000000.00000000.00000000 0xfc000000 252.0.0.0
/7 11111110.00000000.00000000.00000000 0xfe000000 254.0.0.0
/8 11111111.00000000.00000000.00000000 0xff000000 255.0.0.0
/9 11111111.10000000.00000000.00000000 0xff800000 255.128.0.0
/10 11111111.11000000.00000000.00000000 0xffc00000 255.192.0.0
/11 11111111.11100000.00000000.00000000 0xffe00000 255.224.0.0
/12 11111111.11110000.00000000.00000000 0xfff00000 255.240.0.0
/13 11111111.11111000.00000000.00000000 0xfff80000 255.248.0.0
/14 11111111.11111100.00000000.00000000 0xfffc0000 255.252.0.0
/15 11111111.11111110.00000000.00000000 0xfffe0000 255.254.0.0
/16 11111111.11111111.00000000.00000000 0xffff0000 255.255.0.0
/17 11111111.11111111.10000000.00000000 0xffff8000 255.255.128.0
/18 11111111.11111111.11000000.00000000 0xffffc000 255.255.192.0
/19 11111111.11111111.11100000.00000000 0xffffe000 255.255.224.0
/20 11111111.11111111.11110000.00000000 0xfffff000 255.255.240.0
/21 11111111.11111111.11111000.00000000 0xfffff800 255.255.248.0
/22 11111111.11111111.11111100.00000000 0xfffffc00 255.255.252.0
/23 11111111.11111111.11111110.00000000 0xfffffe00 255.255.254.0
/24 11111111.11111111.11111111.00000000 0xffffff00 255.255.255.0
/25 11111111.11111111.11111111.10000000 0xffffff80 255.255.255.128
/26 11111111.11111111.11111111.11000000 0xffffffc0 255.255.255.192
/27 11111111.11111111.11111111.11100000 0xffffffe0 255.255.255.224
/28 11111111.11111111.11111111.11110000 0xfffffff0 255.255.255.240
/29 11111111.11111111.11111111.11111000 0xfffffff8 255.255.255.248
/30 11111111.11111111.11111111.11111100 0xfffffffc 255.255.255.252
/31 11111111.11111111.11111111.11111110 0xfffffffe 255.255.255.254
/32 11111111.11111111.11111111.11111111 0xffffffff 255.255.255.255

SI prefixes

An SI prefix is a name that precedes a basic unit of measure to indicate a decimal multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol.

Name Zeichen Potenz Faktor
Yotta Y 1024 1.000.000.000.000.000.000.000.000
Zetta Z 1021 1.000.000.000.000.000.000.000
Exa E 1018 1.000.000.000.000.000.000
Peta P 1015 1.000.000.000.000.000
Tera T 1012 1.000.000.000.000
Giga G 109 1.000.000.000
Mega M 106 1.000.000
Kilo k 103 1.000
Hekto h 102 100
Deka da 101 10
Dezi d 10-1 0,1
Zenti c 10-2 0,01
Milli m 10-3 0,001
Mikro µ 10-6 0,000.001
Nano n 10-9 0,000.000.001
Pico p 10-12 0,000.000.000.001
Femto f 10-15 0,000.000.000.000.001
Atto a 10-18 0,000.000.000.000.000.001
Zepto z 10-21 0,000.000.000.000.000.000.001
Yocto y 10-24 0,000.000.000.000.000.000.000.001