nmap(1) -sT -PN -n
Network exploration tool and security / port scanner
-sT (TCP connect scan) .
    TCP connect scan is the default TCP scan type when SYN scan is not an option. This is the case when a
    user does not have raw packet privileges or is scanning IPv6 networks. Instead of writing raw packets
    as most other scan types do, Nmap asks the underlying operating system to establish a connection with
    the target machine and port by issuing the connect system call. This is the same high-level system
    call that web browsers, P2P clients, and most other network-enabled applications use to establish a
    connection. It is part of a programming interface known as the Berkeley Sockets API. Rather than read
    raw packet responses off the wire, Nmap uses this API to obtain status information on each connection

    When SYN scan is available, it is usually a better choice. Nmap has less control over the high level
    connect call than with raw packets, making it less efficient. The system call completes connections
    to open target ports rather than performing the half-open reset that SYN scan does. Not only does
    this take longer and require more packets to obtain the same information, but target machines are
    more likely to log the connection. A decent IDS will catch either, but most machines have no such
    alarm system. Many services on your average Unix system will add a note to syslog, and sometimes a
    cryptic error message, when Nmap connects and then closes the connection without sending data. Truly
    pathetic services crash when this happens, though that is uncommon. An administrator who sees a bunch
    of connection attempts in her logs from a single system should know that she has been connect
-PN (No ping) .
    This option skips the Nmap discovery stage altogether. Normally, Nmap uses this stage to determine
    active machines for heavier scanning. By default, Nmap only performs heavy probing such as port
    scans, version detection, or OS detection against hosts that are found to be up. Disabling host
    discovery with -PN causes Nmap to attempt the requested scanning functions against every target IP
    address specified. So if a class B sized target address space (/16) is specified on the command line,
    all 65,536 IP addresses are scanned. Proper host discovery is skipped as with the list scan, but
    instead of stopping and printing the target list, Nmap continues to perform requested functions as if
    each target IP is active. To skip ping scan and port scan, while still allowing NSE to run, use the
    two options -PN -sP together.

    For machines on a local ethernet network, ARP scanning will still be performed (unless --send-ip is
    specified) because Nmap needs MAC addresses to further scan target hosts. This option flag used to be
    P0 (uses zero), but was renamed to avoid confusion with protocol ping´s PO (uses the letter O) flag.
-n (No DNS resolution) .
    Tells Nmap to never do reverse DNS resolution on the active IP addresses it finds. Since DNS can be
    slow even with Nmap´s built-in parallel stub resolver, this option can slash scanning times.
source manpages: nmap