Welcome
First of all, may I welcome you to my site. My name is Chris and I'm from the UK and work as a Systems Engineer for Cisco. This blog was initially created to post up my subnetting technique but has now got more stuff to do with attaining Cisco certifications. Either way I really hope that the content is sufficent for your needs and I look forward to hearing your feedback. If you find that the content really helps you please feel free to donate using the PayPal link on the right.
To view the index of all my articles please click here.
Index
Tuesday, 9 June 2009
Follow the links below to find the main articles on this site:
95th Percentile - How ISPs charge
Access Lists vs Prefix Lists
Cabling Cisco devices
Calculate Multicast MAC Address
CCIE - Warm-Up Phase - BGP
CCIE - Warm-Up Phase - Multicast
CCIE - Warm-Up Phase - OSPF
CCNP Training in the UK
Enhanced Interior Gateway Protocol (EIGRP)
Frame Relay - CIR, Bc, Tc
Frame Relay - BECN/FECN
GNS3 Configuration Guide (Linux)
GNS3 Configuration Guide (Windows)
GNS3 PEMU Configuration Guide
GNS3 VPCS Configuration Guide
How to calculate a wildcard mask
"ip classless" versus "no ip classless"
MPLS Command Memoriser Review
Multicast RPF
NAT In A Nutshell
OSPF and Frame Relay
PPP Authentication - AAA
PPP Authentication - PAP and CHAP
PPPoE Server and Client
QoS - Bandwidth, Bandwidth Percent, Bandwidth Remaining Percent
RIP - Routing Information Protocol
RIP - Adjusting timers
Route Maps and Access Lists
Route Summarization - Easy Networks
Route Summarization - Complex Networks
Summary Route to Null 0 - Routing Loop Prevention
Subnetting Made Easy - Critically Acclaimed!
VLAN Trunking - Which combination forms a trunk?
Vyatta on VM Workstation
Posted byChris Bloomfield at 13:41 0 comments
Labels: best ccnp training, best subnetting guide, ccnp training uk, Cisco, gns3, index, Subnet, subnetting made easy
How to calculate the Wildcard Mask
Wednesday, 12 December 2007
This is really simple.
Simply subtract your mask from 255.255.255.255 to get your wildcard mask.
Example:
The wilcard mask of /26 is:
255.255.255.255 - 255.255.255.192 = 0.0.0.63
The wilcard mask of /19 is:
255.255.255.255 - 255.255.224.0 = 0.0.31.255
The wildcard mask of /12 is:
255.255.255.255 - 255.240.0.0 = 0.15.255.255
There is an alternative way of calculating your wildcard mask which follows on from the Subnetting Made Easy post about boundaries. If you do not understand what I mean about boundaries please read that post.
For example, if you had a /28, your next boundary is /32 so 32 - 28 = 4 and 24 = 16. If we subtract 1 from the block size it gives us 15 which is what we put into our wildcard mask in the octet that we are subnetting in. All octets before the one we are subnetting in are 0 and all octets after the octet we are subnetting in should be 255.
Let's follow that again:
1. /28 gives us 15 in the last octet (i.e. 32 - 28 = 4 and 24 - 1 = 15)
2. All octets before that octet have to be 0 (i.e. 1st, 2nd, and 3rd octets)
3. All octets after that octet have to be 255 (not applicable here as /28 is in the last octet)
4. We therefore have 0.0.0.15 as our wildcard mask
Let's do it with a /21:
1. /21 gives us 7 in the third octet (i.e. 24 - 21 = 3 and 23 - 1 = 7)
2. All octets before that octet have to be 0 (i.e. 1st and 2nd octets)
3. All octets after that octet have to be 255 (i.e. the 4th octet)
4. We therefore have 0.0.7.255 as our wildcard mask
Let's do it with a /9:
1. /9 gives us 127 in the second octet (i.e. 16 - 9 = 7 and 27 - 1 = 127
2. All octets before that octet have to be 0 (i.e. the 1st octet)
3. All octets after that octet have to be 255 (i.e. the 3rd and 4th octet)
4. We therefore have 0.127.255.255 as our wildcard mask
HTH,
Chris
Posted byChris Bloomfield at 11:36 25 comments
Labels: block, mask, size, Subnet, Subnetting, wildcard
