![]() ![]() In the third case above (packet destined to 125.95.39.68), the /32 static route is more specific than the /28 static, and /24 directly connected, so the /32 gets chosen. In the second case above (packet destined to 125.95.39.65), the /28 static route is more specific than the /24 directly connected route, so /28 is chosen. In all cases, what determines which route will be selected follows these rules: What governs this behavior is the rules of Overlapping routes. 125.95.39.68, this packet matches all three routes, and will be forwarded out Interface Y 125.95.39.65, this packet matches two routes, and will be forwarded out Interface Z 125.95.39.55, this packet matches only one route and will be forwarded out Interface X In this case, should the router receives a packet destined to. It is common, and perfectly acceptable, for a Routing Table to have entries that overlap. So in essence, you have three overlapping routes. Let us now imagine an administrator configures the Router for two Static Routes that claim:īoth of these networks contain IP addresses in the same network as the one connected to Interface X. 255), the Router will forward the packet out Interface X. Should this same Router now come across a packet destined to any other IP address in the network (125.95.39. Should this same Router now come across a packet destined to the to 125.95.39.129 (the Router's own interface X IP address), the Router will forward the packet to its CPU for processing. Therefore, the Router will add to its Routing Table that the Network 125.95.39.0/24 maps to Interface X. If a router's interface X is configured with the IP address 125.95.39.129, with a mask of 255.255.255.0, the Router can use the technique in this post to determine that the Network (or Subnet) this IP address resides in is 125.95.39.0/24. A Static Route is a mapping learned by an administrator manually telling the Router about the location of a particular network.A Directly Connected route is a mapping learned due to an interface's configuration.The two which are relevant to your questions are Directly Connected Routes or Static Routes. There are multiple ways a Router can learn entries for its Routing Table. A Routing Table contains many mappings of either a network to an interface or a network to a next-hop IP. Router's populate Routing Tables (which you are calling a Forwarding Table, the terms are synonymous). And to answer them properly we have to explore Routers and how they function. My thinking is that, in the case of #1, we do an exact match between an incoming packet's destination address and that forwarding table entry and only forward on interface x if they're exactly the same.Īssuming that my answer to #1 is correct, for #3, would we forward a packet on i only if the packet's destination is not 125.95.39.60, which is more specific than 125.95.39.0? What if the forwarding table also has 125.95.39.0 and is associated with mask m and interface i? ![]() What does it mean if the same IP address is mapped to two different subnet masks in the forwarding table? Accordingly, each entry also maps to a different interface. Let's say that there is another address, 125.95.39.60 in the router's forwarding table, but it is in the forwarding table twice, and both times, it has a different subnet mask (let's call them y and z) associated with it. If the address 125.95.39.129 is in a router's forwarding table (and maps to interface x), does it imply that only packets whose destination is exactly 125.95.39.129 will be forwarded on x? Or, will we have to use the technique in this post to find the subnet to which this host belongs, and then any packet whose destination belongs to that subnet will be forwarded on x? I have three questions about the relationship between IP addresses, subnet masks, and forwarding tables:
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