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There is a network as follows:DHCP server - Subnet A - 10.10.10.65 - Router - 10.10.10.93 - Subnet B5 Client computers in Subnet A:IP address: 10.10.10.66 - 10.10.10.70Subnet mask: 255.255.255.192Default gateway: 10.10.10.65Server in Subnet B:IP address: 10.10.10.94Subnet mask: 255.255.255.240Default gateway: 10.10.10.93Clients cannot connect to the server in Subnet B unless the subnet mask ofthe server is changed to 255.255.255.192Why is that? Isn't it the router's job to forward the packets from onesubnet to the [email protected] 23:29. While it's hard to say without more information, your first problem isthat your two subnets overlap. IOW, subnet B is addresses10.10.10.92-10.10.10.107, which is wholly within subnet A(10.10.10.64-10.10.10.127). That basically needs to be fixed.That you're getting any communication between the clients on A and theserver on B suggests that you also have the two subnets on a singlephysical network.
When you change the subnet mask on B to255.255.255.192, you're basically including the clients, which, ifyour on the same physical LAN, allows them to talk. While there aresituations where having more than one subnet on a single LAN is(debatably) useful or desirable, it's more likely to be a networkdesign error, or a misconstrued (and incorrect) attempt to partitionthe network for security reasons.News Reader03.07.08 10:01. As indicated by the other respondent, your key issue is the addressoverlap resulting from your erroneous network mask assignments.It's important to understand that the first routing decision is made bythe 'host'. A host applies its network mask to its own IP address andthe destination host IP address, then performs a comparison to determinewhether the destination host is directly reachable, or whether thepacket needs to be forwarded to the router.When host addresses are mis-assigned due to erroneous masks, a host mayerroneously conclude that a destination host is directly reachable, andattempt to resolve its IP address into a MAC address via ARP (whichfails), rather than forwarding the packet to the router.
You can see from the default subnet mask shown in the preceding table that the first octet is set to all 1 s (dotted decimal 255). Recall that a network ID cannot be. This free online IP subnet calculator covers both IPv4 and IPv6 protocols, providing information such as IP address, network address, subnet mask, IP range, and more. Also, explore. Below is a table providing typical subnets for IPv4.
This couldoccur on the initiator or the responder side depending on the erroneousconfiguration.The selection of masks should be driven by the potential number of hoststo be assigned to each network.Typically, the first or last assignable host address available for eachnetwork will be used for the gateway(s) which can lessen confusion forthose unfamiliar with your addressing scheme.Best Regards,News ReaderBarry Margolin03.07.08 19:44. Wrote in messagenews:5bed34cb-ae13-4e58-90cb-73d5cc6274d1@x41g2000hsb.googlegroups.com.On Jul 3, 12:37 am, 'eager' wrote: There is a network as follows: DHCP server - Subnet A - 10.10.10.65 - Router - 10.10.10.93 - Subnet B 5 Client computers in Subnet A: IP address: 10.10.10.66 - 10.10.10.70 Subnet mask: 255.255.255.192 Default gateway: 10.10.10.65 Server in Subnet B: IP address: 10.10.10.94 Subnet mask: 255.255.255.240 Default gateway: 10.10.10.93 Clients cannot connect to the server in Subnet B unless the subnet mask of the server is changed to 255.255.255.192 Why is that? Isn't it the router's job to forward the packets from one subnet to the other?While it's hard to say without more information,there is no more INFO, it's a question from 70-291 exam.your first problem is that your two subnets overlap.What do you mean by ' two subnets overlap'?
- they are in totally differentbroadcast domains.IOW, subnet B is addresses 10.10.10.92-10.10.10.107,No idea where you got that from, because based on the server INFO, the IPrange of the subnet B should be 10.10.10.80 - 10.10.10.95That IP range does not overlap with the scope 10.10.10.66 - 10.10.10.70 ofthe clients in subnet A.Since the gateway is configured properly on both subnets, according to therouting table I do not see why there will be no connectivity between theclients and the server.which is wholly within subnet A (10.10.10.64-10.10.10.127). 'News Reader' wrote in messagenews:[email protected]. eager wrote: There is a network as follows: DHCP server - Subnet A - 10.10.10.65 - Router - 10.10.10.93 - Subnet B 5 Client computers in Subnet A: IP address: 10.10.10.66 - 10.10.10.70 Subnet mask: 255.255.255.192 Default gateway: 10.10.10.65 Server in Subnet B: IP address: 10.10.10.94 Subnet mask: 255.255.255.240 Default gateway: 10.10.10.93 Clients cannot connect to the server in Subnet B unless the subnet mask of the server is changed to 255.255.255.192 Why is that? In article, 'eager' wrote:Subnet A's subnet mask is 255.255.255.192, which means it's a block of64 addresses.
So the range is 10.10.10.64 - 10.10.10.127. Subnet B'saddress range 10.10.10.80 - 10.10.10.95 overlaps that.P.S. Please use proper prefixing of quotes so that your replies can bedistinguished from the message you're replying to. It's really hardreading your message because of this.-Barry Margolin,Arlington, MA. PLEASE don't copy me on replies, I'll read them in the group.eager03.07.08 20:56.
'News Reader' wrote in messagenews:[email protected] you for your reply, but I am afraid you are missing the whole pointhere:After 'fixing' the problem by changing the subnet mask, there is a betteroverlapping of the subnets.What would you say to that?Anyway, I am aware of the 'route print' as I am aware of 'arp -a', but letsnot confuse routing with routes and routing tables with broadcasting andunicasting.All I need to know is why making both subnet masks the same will establishthe connectivity.ThanksNews Reader03.07.08 21:54. Eager wrote: Sorry, I will keep it in mind. Anyway, this overlapping theory seems nonsense to me. As I mentioned it, after 'fixing' the subnet, there is even a better overlap:)Perhaps you have fixed the wrong subnet, and or used the wrong mask.Please read this entire post before responding.You initially stated ( 1:37 AM):'Clients cannot connect to the server in Subnet B unless the subnet maskof the server is changed to 255.255.255.192'Robert offered an explanation ( 2:29 AM):'That you're getting any communication between the clients on A and theserver on B suggests that you also have the two subnets on a singlephysical network.' But did NOT say you were to change the subnet B mask to'255.255.255.192'.Barry indicated ( 10:44 PM):'If there's some reason you can't fix the subnet masks on Subnet A.' Have you corrected 'Subnet A', and used an appropriate mask?e.g:Subnet A with a mask of:Network ID: 10.10.10.64Host range: 10.10.10.65 - 10.10.10.78Broadcast address: 10.10.10.79Subnet B with a mask of:Network ID: 10.10.10.80Host range: 10.10.10.81 - 10.10.10.94Broadcast address: 10.10.10.95Best Regards,News Readereager03.07.08 22:01. In article, 'eager' wrote: There is a network as follows: DHCP server - Subnet A - 10.10.10.65 - Router - 10.10.10.93 - Subnet B 5 Client computers in Subnet A: IP address: 10.10.10.66 - 10.10.10.70 Subnet mask: 255.255.255.192 Default gateway: 10.10.10.65 Server in Subnet B: IP address: 10.10.10.94 Subnet mask: 255.255.255.240 Default gateway: 10.10.10.93 Clients cannot connect to the server in Subnet B unless the subnet mask of the server is changed to 255.255.255.192Something's very confusing here.
At the very top you have the DHCPserver in Subnet A, but now you say the server is in Subnet B. Which isit?And the solution of changing the subnet mask on the server in Subnet Bwould only make sense if the device in between was a hub or switch, nota router. For a router, the fix should be to correct the subnet mask ofSubnet A, as I said earlier.I can't figure out why that change fixes things, and I've been managingTCP/IP networks (including a nationwide ISP backbone) for 20 years.-Barry Margolin,Arlington, MA.
PLEASE don't copy me on replies, I'll read them in the group.PacketU04.07.08 13:35. I have been following this for a while silently. However, this appears tobe train wreck. It seams as you do not understand the relation ship betweenIP addressing, subnet masks and how it pertains to routing. Thank you for your reply, but I am afraid you are missing the whole point here: After 'fixing' the problem by changing the subnet mask, there is a better overlapping of the subnets.
What would you say to that?I would say that overlapping of subnets is bad. I don't know a how therecan be better or any gray are for this. That is sort of like saying thatbinary has three values (0,1 and 2). That is not the case. If there is anyoverlap in subnet, you better know what you are doing to address it properlywith nat. Also, there are concerns like proxy arp that change the behaviorof inconsistent mask on the router and hosts.
Anyway, I am aware of the 'route print' as I am aware of 'arp -a', but lets not confuse routing with routes and routing tables with broadcasting and unicasting.Actually, these are all related. Route print shows the routing table. Arpalso lends clues to if you are doing proxy arp etc.
Route print is thelocal routing table. This defines the broadcast. Unicast is anything thatfalls within this range as well as anything that goes out non multicastdestination. However, in some cases this can be converted to a directedbroadcast by destination router. You cannot tell that though. All of theseare the concepts that go together to understand how a frame is built, andwhat the destination mac address is (the real destination, or the router) All I need to know is why making both subnet masks the same will establish the connectivity.
ThanksWhat you need to understand is how IP functions. If you understand this,this is a very simple thing to troubleshoot and understand for you. You canuse wireshark. Understanding that the communication is bidirectional, doeseach host arp for the destination ip or the gateway?
Check in the reversedirection as well. If it arps for the destination, the router may respondwith a proxy arp. The frame can be build with a destination IP of thedestination and the destination mac of the router. Or if everthing isproperly configured, the hosts should arp for the default gateways and thepackets would be built as such.
In nearly every case like this, the issueis only in one direction.There have been many people took time to answer the questions to the bestof anyones abilities. They are correct in response based on what you havetold them.
I would suggest that if you are interested in understanding thisthat you do two things. Get a copy of TCP/IP unleashed by Tim Parker.This book is great. The next thing is to start looking at how thisstuff works in Wireshark.TCP/IP alone is not very complicated once you understand it. Well, I guessthat can be said for anything.eager04.07.08 17:24. In article,Barry Margolin wrote: It's an actual question from Microsoft 70-291 exam.You keep saying that as if '70-291' is supposed to mean something to us.' Impress us' might be more accurate than 'mean something to us.'
Might have clues about it. One hit iswhich says in part:The Microsoft Certified Systems Administrator (MCSA) on WindowsServer 2003 credential is intended for IT professionals whowork in the typically complex computing environment of mediumto large companies.
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The following summarizes how networks, subnetworks and hosts are identified in the TCP/IP protocol. An IP address is first divided between networks and hosts. The host bits are further divided between subnets and hosts. See subnet mask and subnet.
Summary of Classes A, B & C
There are very few Class A networks, but each one can contain thousands of subnets and hosts (defined in 24 bits).
CLASS B SUBNET MASKS (LARGE NETWORKS)How 16Bits Are ---Maximum---Subnet Mask Divided Subnets/Hosts 255.255.192.0 2/14 2 16,382 255.255.224.0 3/13 6 8,190 255.255.240.0 4/12 14 4,094 255.255.248.0 5/11 30 2,046 255.255.252.0 6/10 62 1,022 255.255.254.0 7/9 126 510 255.255.255.0 8/8 254 254 255.255.255.128 9/7 510 126 255.255.255.192 10/6 1,022 62 255.255.255.224 11/5 2,046 30 255.255.255.240 12/4 4,094 14 255.255.255.248 13/3 8,190 6 255.255.255.252 14/2 16,382 2 CLASS C SUBNET MASKS (SMALL NETWORKS)How 8Bits Are ---Maximum---Subnet Mask Divided Subnets/Hosts 255.255.255.192 2/6 2 62 255.255.224.224 3/5 6 30 255.255.240.240 4/4 14 14 255.255.248.248 5/3 30 6 255.255.252.252 6/2 62 2Class C Subnet Masks
The last 8 bits of the mask identify subnets and hosts. Network administrators have to determine how best to construct the network: more subnets and fewer hosts or more hosts and fewer subnets.
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