How to block/unblock an IP from Linux?

In order to block an IP on your Linux server you need to use iptables tools (administration tool for IPv4 packet filtering and NAT) and netfilter firewall. First you need to log into shell as root user. To block an IP address you need to type the iptables command as follows:

Syntax to block an IP address under Linux

iptables -A INPUT -s IP-ADDRESS -j DROP

Replace IP-ADDRESS with your actual IP address. For example, if you wish to block an ip address 65.55.44.100 for whatever reason then type the command as follows:

# iptables -A INPUT -s 65.55.44.100 -j DROP

If you have IP tables firewall script, add the above rule to your script.
If you just want to block access to one port from an ip 65.55.44.100 to port 25 then type command:

# iptables -A INPUT -s 65.55.44.100 -p tcp --destination-port 25 -j DROP

The above rule will drop all packets coming from IP 65.55.44.100 to port mail server port 25.

CentOS / RHEL / Fedora Block An IP And Save It To Config File

Type the following two command:
# iptables -A INPUT -s 65.55.44.100 -j DROP
# service iptables save

How Do I Unblock An IP Address?

Use the following syntax (the -d options deletes the rule from table):
# iptables -D INPUT -s xx.xxx.xx.xx -j DROP
# iptables -D INPUT -s 65.55.44.100 -j DROP
# service iptables save

Copy and paste text with vi or vim

The ability to duplicate text in an editor can be handy. vi and vim have several useful copy and paste commands.

The command ‘Y’ or ‘yy’ copies (yanks) one or more lines. To copy one line, two lines, 10 lines, and all lines to the end of the file, respectively:

Y 2Y
10Y
yG

To paste the text contained in the buffer above (uppercase P) or below the current cursor position (lowercase p), respectively:

P p

It is also possible to yank text within a line. The following commands yank text from the current cursor position to the end of the word and the end of the line, respectively:

yw y$


The same commands paste the text within a line. Lower case p pastes after the cursor position and upper case P pastes before.

Paste will also work with deleted text, either lines or parts of lines. Be careful not to execute any other commands prior to pasting as this will empty the buffer.