[CentOS] Inquiry:Problem with nmap

Dear All
Can you please do me favor and let me know how can I solve my problem with
nmap , as described below :
"My CentOS server can ping the network element , but when I try to scan its
open ports to see which ones are open at now the nmap cannot distinguish
them and returned as they are being filtered :
#nmap -sS -p 20-80 172.18.0.1
Can you please let me know how can I modify my nmap command to get through ?
Thank you in advance
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