![]() This has got to be the most concise and least brain cell damaging explanation I've come across to figure out the subnets when using prefix notation." via Cisco Forum "Great, Keep it up! Your easy and quick method helped me a lot to pass CCNA. Great job." via Cisco Forum "I wanted to thank you for posting this. Using your technique I'm able to answer subnetting questions in 20 sec in my head. Guys, I recommend this subnetting tutorial." Forum Admin, "I wanted to thank the original poster for this information.reading this finally made it click and solved the mystery of subnetting for me and was instrumental in passing the CCNA yesterday." via Cisco Forum "I just passed the CCENT test. I use GNS3 and Dynamips Greaaaaaaaaaaaat" Fayda, GNS3 Configuration Guide comments "Chris, I just visited your blog and I want to congratulate you on your effort. Good work, if you have some tips for CCNA please post. This is completely invaluable information." Brad, GNS3 Configuration Guide comments "Hi, just tested it. Thanks so much for not only the tip, but the detailed tutorial on how to setup and actually use GNS3. I kept thinking to myself, if only there was a better way. ![]() I was going to actually buy some lame ass simulator software or buy a bunch of cisco equipment to prepare for my CCNA. "Can I just say, I am absolutely in LOVE with you and this blog right now. Only the remaining 5 bits of the 9 top bits will overlap (as the last 23 bits are used for the second half of Multi-cast MAC address) Regarding your point about 512 possible overlaps, I think we only have to worry about the 224.0.0.0 - 239.255.255.255 range when it comes to multi-casts as it's the reserved range for this purpose. Hi, thanks for explaining the MAC calculations. Basically any IP address with 40.35.1 or 168.35.1 as the last three octets carry the same multicast MAC address so you have potentially 32 addresses with the same multicast MAC address! So 192.168.35.1 has a multicast MAC address of 01-00-5E-28-23-01.Ĭan you spot an issue here? Hopefully you can. Our multicast MAC address is now 01-00-5E-28-23-01 In this example the value is 1 which equals 0x01. To calculate the value of ZZ take the fourth octet and convert it to hex. Our multicast MAC address is now 01-00-5E-28-23-ZZĤ. In this example the value is 35 which equals 0x23. To calculate the value of YY take the third octet and convert it to hex. Our multicast MAC address is now 01-00-5E-28-YY-ZZģ. In this example, the value of 168 is greater than 128 so we subtract 128 from 168 to give us a value of 40. If the value of the second octet is greater than 128 then subtract 128 from the second octet. To calculate the value of XX take the second octet. Start with a half-filled multicast MAC address of 01-00-5E-XX-YY-ZZĢ. ![]() Let us try and convert 192.168.35.1 to a multicast MAC addressġ. Therefore it must follow that a value of 6 in the second octet must be the same as 134 in the second octet as the high-order bit (i.e. ![]() If you think about this we are not using the high order bit in the second octet which carries a value of 128. To do this we need to convert the last 23 bits of the IP address in question. The first half of a multicast MAC address is 01-00-5E so we need to work out the second half. Each class has a range of valid IP addresses.Īddress Range of Class A - 1.0.0.1 to 126.255.255.254 (supports 16 million hosts on each of 127 networks).Ĭlass B - 128.1.0.1 to 191.255.255.254 (65,000 hosts on each of 16,000 networks).Ĭlass C - 192.0.1.1 to 223.255.254.254 (254 hosts on each of 2 million networks).Ĭlass D - 224.0.0.0 to 239.255.255.255 (address range reserved for multicast groups).Ĭlass E - 240.0.0.0 to 254.255.255.254 (reserved for future use, research and development purposes).It's been a while but here's a quick post on how to calculate a multicast MAC address from an IP address. We can see two things: all host bits are zeroes in a network address, in a broadcast address they are all set.įirst bits determine the class of your network from A to E. This free online IPv4 subnet calculator also can be used as a teaching tool and presents the subnetting results as easy-to-understand binary values. The wildcard is the inverse netmask used for access control lists (ACL's) in Cisco routers. Cisco wildcard mask, host range and quantity of hosts. ![]()
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