I realize Marc already answered this correctly so not much point in my response, however using the same logic I derived the same answer.
10.1.32.2/24 10.1.33.2/24 10.1.34.2/24 10.1.35.2/24 128|64|32|16|8|4|2|1 00100000.00000010 00100001.00000010 00100010.00000010 00100011.00000010 ------------------- 00000011.00000000 = 0.0.3.0 00100000.00000010 00100001.00000010 00100010.00000010 00100011.00000010 00100011.00000011 ------------------- 00000011.00000001 = 0.0.3.1 If you write an article on this please send the link. Cheers! Thank you, Steve Di Bias Network Engineer - Information Systems Valley Health System - Las Vegas Office - 702- 369-7594 Cell - 702-241-1801 [email protected] -----Original Message----- From: Terry Slattery [mailto:[email protected]] Sent: Wednesday, April 13, 2011 5:44 AM To: Di Bias, Steve Cc: Jay Taylor; <[email protected]> Subject: Re: [OSL | CCIE_RS] offset-list and wild card mask Steve, I thought that you might have hit 'send' a bit too quickly. But it gave me a great opportunity to continue the thread to show what happens as you try to match other things - in this case matching all hosts in each subnet or matching all subnets in the second octet. Your example would be good for matching specific subnets in a routing table, where the last octet would be '0'. My example is better for matching devices in an ACL. As a continuation of the exercise, and to make sure everyone understands wildcard masks, let's create a more real-world scenario, similar to what you might find on a CCIE exam. Task: Configure OSPF on a router with four interfaces: 10.1.32.2/24 10.1.33.2/24 10.1.34.2/24 10.1.35.2/24 What wildcard mask will match only those interfaces in a single OSPF network configuration statement? Now let's say that there's an adjacent router and you're configuring HSRP between them. How would the network statement have to change for use in the adjacent router that uses the .3 host address (i.e. 10.1.32.3/24)? Is there a wildcard mask that will work for both routers, so that you have an ospf configuration that can be shared between the two routers? You can't arrive at the right answer to the above by inverting the subnet mask. I'll may have to write about this subject in a blog post... especially since my quick Google search came up with so much inaccurate information on the subject. -tcs On 4/12/11 9:38 PM, Di Bias, Steve wrote: > Terry I realized that I put a zero there after I hit send, I just didn't send > another email correcting myself. The point I was making was that you were > correct, and I was attempting to show that with the wildcard mask. > > //s > > Terry Slattery<[email protected]> wrote: > > > Steve, > > That matches the 10.20.4.0 subnet and 10.30.4.0 subnet, but it won't match any > hosts on either subnet, because the last octet is 0 and the wild-card bits are > zero (must match). > > I was looking for a wildcard mask of 0.10.0.255 to match all hosts on either > subnet. > > If you went further, you could have also come up with 0.255.0.255, to match > 10.x.4.x. > > -tcs > > On 4/12/11 12:15 AM, Di Bias, Steve wrote: >> Terry is right, I may have jumped the gun with the inverse mask statement >> (although it's mentioned this way in many documents). For Terry's experiment >> let's assume we want to match the voice vlan for buildings 20 and 30 >> (10.20.4.0 and 10.30.4.0). By breaking this down into binary and using >> AND/OR logic we can easily come up with our answer >> >> 00001010.00010100.00000100.0000000 >> 00001010.00011110.00000100.0000000 >> ---------------------------------- >> 00001010.00010100.00000100.0000000 = 10.20.4.0 >> >> 00001010.00010100.00000100.0000000 >> 00001010.00011110.00000100.0000000 >> ---------------------------------- >> 00000000.00001010.00000000.0000000 = 0.10.0.0 >> >> So the "wildcard" mask to match both buildings would be 0.10.0.0 >> >> Cheers! >> >> Thank you. >> >> Steve Di Bias >> Network Engineer - Information Systems >> Valley Health System - Las Vegas >> Office - 702- 369-7594 >> Cell - 702-241-1801 >> [email protected] >> >> -----Original Message----- >> From: [email protected] >> [mailto:[email protected]] On Behalf Of Terry Slattery >> Sent: Monday, April 11, 2011 6:58 PM >> To: Jay Taylor >> Cc:<[email protected]> >> Subject: Re: [OSL | CCIE_RS] offset-list and wild card mask >> >> Good answer, Jay. For everyone who thinks that the wild-card mask is the >> opposite of the subnet mask... >> >> I have a set of subnets that I need to match. The first octet is 10. The >> second octet is a building number. The third octet identifies the subnet in >> each building, and is "4" for the voice subnet, which is what I want to >> match. >> >> Build a wild-card mask that matches >> 10.x.4.x >> >> Is it the inverse of the subnet mask? >> >> -tcs >> >> On 7/22/64 2:59 PM, Jay Taylor wrote: >>> Offset list is used to increment the metric of certain routes. >>> >>> In a wildcard mask a binary 0 means the bit must match and a binary 1 >>> means it does not have to match. This is reverse logic compared to a >>> normal subnet mask. Also, unlike a subnet mask the 1's and 0's do not >>> need to be contiguous. >>> >>> >>> On Mon, Apr 11, 2011 at 11:04 AM, Uli<[email protected]> wrote: >>> >>>> Hi Expert, >>>> >>>> Does anyone can explain to me about offset-list as I kind of confused >>>> with it. also, in my opinion that wild card mask is reverse of subnet >>>> mask, but someone told me it isn't ? >>>> >>>> >>>> Regards >>>> _______________________________________________ >>>> For more information regarding industry leading CCIE Lab training, >>>> please visit www.ipexpert.com >>>> >>> >>> >>> >> > > -- > Terry Slattery CCIE# 1026 > > > > UHS Confidentiality Notice: This e-mail message, including any attachments, > is for the sole use of the intended recipient (s) and may contain > confidential and privileged information. Any unauthorized review, use, > disclosure or distribution of this information is prohibited. If this was > sent to you in error, please notify the sender by reply e-mail and destroy > all copies of the original message. -- Terry Slattery CCIE# 1026 UHS Confidentiality Notice: This e-mail message, including any attachments, is for the sole use of the intended recipient (s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure or distribution of this information is prohibited. If this was sent to you in error, please notify the sender by reply e-mail and destroy all copies of the original message. _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com
