In the following, I assume you're using a 255.255.255.0 subnet mask on the subnets…
Think of destination interface Null0 as the "black hole" for network traffic on a router: traffic goes in, but it doesn't come out. It just disappears. So throw anything you don’t want, into that interface.
Some of the traffic flows you sampled are just background broadcast noise on the attached interfaces. Specifically, Microsoft Networking clients broadcasting periodically on their respective subnets. Note the destination IPs ending in .255; and protocol 11 in hexadecimal, or 17 in decimal, which is UDP; and source port and destination port 0089 hex (137 decimal) or 008A hex (138 decimal) on the subnet. The router hears these broadcasts, but if there's no "IP helper-address" on the router interface (or if there is, but "no ip forward-protocol udp" commands are elsewhere in the router config to keep NETBIOS broadcasts from propagating outside each subnet) then the router does not forward them.
There's another UDP broadcast listed on your VLAN 89going to port 7500 decimal (1D4C hex). Not sure what that service is; but since it's a UDP broadcast and “IP helper-address” probably isn't set up to pass it outside of the local subnet, it gets dumped into Null0.
Two different hosts on your VLAN 88 (hosts 172.16.88.121 and .235) are both trying to access TCP port 6667 decimal (1A0B hex) on destination IP address 217.20.115.147. Most likely an ACL is blocking access to that service, whatever it is. This blocking could appear as sending traffic to destination Null0.
Similarly, two hosts on neighboring subnets, 172.16.89.43 and 172.16.90.163, are both trying to access TCP port 4000 decimal (0FA0 hex) on destination IP address 63.246.146.121. Most likely an ACL is blocking access to that service too, whatever it is. Again, the denied traffic is disposed of by sending it to Null0.
On your VLAN 91, I’m guessing here: it looks like either there is an “IP helper-address” pointing to host 172.40.40.16 (address ends in an even number, not odd, so it’s not a broadcast) but maybe a “no ip forward-protocol udp” command is blocking forwarding of UDP port 137 traffic; or, there’s an ACL blocking that kind of traffic from your VLAN 91 host (172.16.91.116) to that destination IP address on that UDP port number. This kind of ACL might be in place to protect against worm- or virus-infected computers from finding UDP 137 hosts that they might be able to compromise. Or maybe it’s a probe of an IP address or subnet that doesn’t exist on your network, and got dumped to Null0.
Looks like VLAN 232 had some HTTP traffic blocked (TCP port 80 decimal, 0050 hex), probably by an ACL.
And IP multicast routing is possibly not enabled on your network, because that one strange-looking IP destination address, 230.0.0.2, is not a Class A, B, or C network number: it’s Class D (224.*.*.* to 239.*.*.*) which is for IP Multicast.
Let me know if any of this makes sense to you. (It's late here, and I'm rambling.)
