What does this mean?

[Chris Hoeschen]

I found this in my logs.  Why are there two different addresses, the source address coming in and the destination address where my box is returning the packet to?   May 16 07:25:17 kernel:IN=eth1 OUT=eth0 SRC="10.20.56.1" DST=192.168.10.17 LEN=56 TOS=0x00 PREC=0x00 TTL=63 ID=10459 PROTO=ICMP TYPE=11 CODE=0 [SRC="{My Ext" IP} DST=63.53.111.64 LEN=80 TOS=0x00 PREC=0x00 TTL=1 ID=13656 PROTO=ICMP TYPE=8 CODE=0 ID=512 SEQ=3072 ]