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Paste the output of "show ip route" from a Cisco IOS XE router and enter a destination address. The explainer lists every route in the table that contains the destination, picks the one with the longest prefix, and shows its next hops, with the binary working. The lookup runs in this page, and the page does not send what you paste to a server. No account is required.
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The explainer is ready. It runs in this page, and the page does not send what you enter to a server.
One row for each route read from the paste, in the order of the paste, with whether it contains the destination and why it wins or loses.
| Line | Route | Next hop | Contains the destination | Result |
|---|
Each step shows the 32 bits in binary and dotted decimal. Network bits are shown in bold and host bits in normal weight where a prefix applies.
A routing table can hold several routes that contain the same destination. The example below holds static routes from a lab router's table. 10.1.1.77 is inside 0.0.0.0/0, 10.0.0.0/8, 10.1.0.0/16 and 10.1.1.0/24. The router uses the route with the longest prefix, here 10.1.1.0/24, because a longer prefix names a smaller block of addresses and so says more about where this destination is. This is the longest prefix match rule.
S* 0.0.0.0/0 [1/0] via 172.16.12.2
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
S 10.0.0.0/8 [1/0] via 172.16.12.2
S 10.1.0.0/16 [1/0] via 172.16.12.6
S 10.1.1.0/24 [1/0] via 172.16.12.2
S 10.1.1.200/32 [1/0] via 172.16.12.6
S 10.2.0.0/16 is directly connected, Null0Route source, administrative distance and metric do not take part in this choice. They decide which route for a given prefix gets installed in the table in the first place. Once routes are installed, only the prefix length decides between them, so a static /16 loses to an OSPF /24 that also contains the destination.
The example is part of a table and has no gateway line, so if you paste it, the explainer calls each answer the best match in this output. The next section explains why.
To practice, copy the example table and decide on paper which route wins for 10.1.1.200, for 10.1.9.9 and for 10.2.3.4. Then paste the table, run each address, and compare your answer with the table of routes.
"show ip route static", "show ip route ospf" and "show ip route ... longer-prefixes" print only part of the table. From that output the explainer can show which of the pasted routes contain the destination and which of them is longest. It cannot show what the router does with the packet, because the full table may hold a longer route, or a default route, that the filter left out.
The explainer checks the paste for three signs that it is filtered: a command line with anything after "show ip route", a "Gateway of last resort" line that names a default route the paste does not list, and an "is subnetted" header that counts more subnets than the paste lists under it. When it finds one, it calls a match the best match in this output, and when no pasted route contains the destination, it gives no answer instead of saying the router has no route. A paste with no gateway line gets the same treatment, because it may not be the whole table. On the lab router the fixtures came from, a longer-prefixes view of 10.1.0.0/16 holds no route for 10.7.7.7, but "show ip route 10.7.7.7" on the same router named 10.0.0.0/8 via 172.16.12.2.
A route contains a destination when the destination's first bits, as many as the prefix length, equal the route's network. The working keeps that many bits of the destination by ANDing it with the prefix mask and compares the result with the network, for the longest containing routes first. A route that does not contain the destination is listed with the first bit where the destination leaves it.
When the winning route has more than one next hop, IOS prints the extra next hops on the lines below the route, and the router shares traffic across them. The explainer lists every next hop and does not predict which one a packet takes. A route to Null0 discards matching packets. A default route, 0.0.0.0/0, contains every address and so wins only when no longer route contains the destination. On the router the fixtures came from, "show ip route 198.18.0.1" printed "% Network not in table" even though the default route forwards that address; the explainer reports the default route as the match.
It reads the IPv4 "show ip route" output captured from IOS XE 17.12.1: the full table and the static, ospf and longer-prefixes variants, with the code legend, the gateway line, the classful "is subnetted" headers and equal-cost next-hop lines. It evaluates routes with the codes C, L, S, S*, O, O IA, O E2. A route line with any other code is read for its prefix only: if that prefix could win, the explainer gives no answer rather than guess. Any line it does not recognize, such as a VRF "Routing Table:" header or the single-route "Routing entry for" output, also stops it from giving an answer, because that line could hold the winning route.
The answer depends on the whole table you pasted, and a routing table is device output. This page never writes device output to a link, so the explainer has no share link and never writes the table or the address to the page's URL. Use Copy results to keep a record.
The explainer looks up one destination in the table you paste. It does not follow a next-hop address to find the outgoing interface, and it does not model policy-based routing or VRFs, and it reads the routing table, not the router's forwarding table (CEF). It reads IPv4 tables only, and only the codes listed above. The output it reads was captured from Cisco IOS XE 17.12.1 running as IOL on Cisco Modeling Labs; other releases and platforms can print tables differently. The working shows the 4 longest containing routes, and the table lists every route.
Sources: RFC 1812, section 5.2.4.3, for longest match route lookup; RFC 4632, section 5.1, for discarding traffic to an aggregate with no more specific route; Cisco, Configure Route Selection for Routers, for administrative distance and metric deciding what is installed and the longest prefix match deciding among installed routes.