This howto will explain you how to capture tcpdump in pcap format and save in log file and then able to read it.
# tcpdump -i eth0 -s0 -w /tmp/capture.pcap
Please find below conversion command for Human readable format for capture.cap file
# tcpdump -qns 0 -X -r /tmp/capture.pcap > /tmp/readable.txt
Now you can open readable.txt to see the log file.
# vi /tmp/readable.txt
tcpdump on specific port.
# tcpdump -ni eth0 port 80