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Java Forum / General / November 2006Join Multicast Group from different subnets
I am trying to get two machines on two different subnets to join the same multicast group. Once they join they periodically send messages to one another via the group, however, right now they don't seem to know of each other. this could be a firewall issue, but I wanted to make sure what I am doing is correct. I try to join the multicast group at "230.0.0.1" per the Java tutorial (http://java.sun.com/docs/books/tutorial/networking/datagrams/broadcasting.html) ....is this address sufficient for my needs? also, what other address could I use...i am not sure what the possibilities are? thanks in advance. > I am trying to get two machines on two different subnets to join the > same multicast group. Once they join they periodically send messages > to one another via the group, however, right now they don't seem to > know of each other. this could be a firewall issue, but I wanted to > make sure what I am doing is correct. You need to configure your router to handle the multicast group. /gordon  Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > On 28 Nov 2006 06:17:35 -0800, Java and Swing wrote > You need to configure your router to handle the multicast group. > > /gordon so what multicast groups (ip's) can i use in general? other than 230.0.0.1? > so what multicast groups (ip's) can i use in general? other than > 230.0.0.1? You can use any class D address (224.0.0.0 - 239.255.255.255), just like the API documentation for MulticastSocket says. But the specific choice of group is not really relevant to your original problem, you still need to enable multicast routing between the subnets. If that's not a workable solution for you, use a multicast tunnel of some kind (try Google). /gordon Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > If that's not a workable solution for you, use a > multicast tunnel of some kind (try Google). not sure if that's what I need. According to one site, "There is a need to limit how long multicast traffic will expand across routers. That's what the TTL is used for. Routers have a TTL threshold assigned to each of its interfaces, and only datagrams with a TTL greater than the interface's threshold are forwarded. " They state.... TTL Scope 0 Restricted to the same host. Won't be output by any interface. 1 Restricted to the same subnet. Won't be forwarded by a router. <32 Restricted to the same site, organization or department. <64 Restricted to the same region. <128 Restricted to the same continent. <255 Unrestricted in scope. Global. So, it seems that I can set the TTL on the datagram to be something like 30, 50, etc to have it get past the router. Link: http://www.oser.org/~oser/ds/node27.html > So, it seems that I can set the TTL on the datagram to be something > like 30, 50, etc to have it get past the router. You can set the TTL to whatever you like but if the router isn't configured to forward multicasts it won't. > You can set the TTL to whatever you like but if the router isn't > configured to forward multicasts it won't. true, actually it seems there is no method to set the TTL anyway. > true, actually it seems there is no method to set the TTL anyway. What do you think MulticastSocket.setTimeToLive() does? /gordon Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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