How to fix MAC Address with IP Address on DHCP server.

Today, I would like to monitor my internet network bandwidth. When my internet network is slow I often have questions from my boss “Why my internet network is slow, who are using a lot of internet bandwidth?”. I cannot answer these questions because I don’t have informations of current network status.

First step, I think I should know which IP Address is using a lot of internet bandwidth and second step I should know who is owner that IP Address.

The answer of the first step , I will use ntop software to answer this question. For the second step, I will fix all MAC Address of Laptop’s wireless card with IP Address on a DHCP Server. And this is my solutions :

1. Get a MAC Address from Client’s laptop such as  A1:B2:C3:A1:B2:C3

2. Add the MAC Address into the file /etc/dhcpd.conf such as

ddns-update-style interim
ignore client-updates

subnet netmask {

   # The range of IP addresses the server
   # will issue to DHCP enabled PC clients
   # booting up on the network


   # Set the amount of time in seconds that
   # a client may keep the IP address

   default-lease-time 86400;
   max-lease-time 86400;

   # Set the default gateway to be used by
   # the PC clients

   option routers;
   # Don't forward DHCP requests from this
   # NIC interface to any other NIC
   # interfaces

   option ip-forwarding off;

   # Set the broadcast address and subnet mask
   # to be used by the DHCP clients

   option broadcast-address;
   option subnet-mask;

   # Set the NTP server to be used by the
   # DHCP clients

   option ntp-servers;

   # Set the DNS server to be used by the
   # DHCP clients

   option domain-name-servers;

   # If you specify a WINS server for your Windows clients,
   # you need to include the following option in the dhcpd.conf file:

   option netbios-name-servers;

   # You can also assign specific IP addresses based on the clients'
   # ethernet MAC address as follows (Host's name is "laser-printer":

   host user-one {
      hardware ethernet A1:B2:C3:A1:B2:C3;
# List an unused interface here
subnet netmask {

3. Check your configuration file is correct :

# /usr/sbin/dhcpd -t -cf /etc/dhcpd.conf

4. Restart your DHCP Service :

# service dhcpd restart

5. Set DHCP relay in your CISCO switch points to your DHCP Server (Assume your IP Address of DHCP Server is

$ conf t
$ conf t > interface vlan 10
$ conf t > ip address
$ conf t > no ip directed-broadcast
$ conf t > ip helper-address
$ conf t > end

6. You can check all IP Addresses which receive from DHCP Server in file /var/lib/dhcp/dhcpd.leases
References :

0 0 vote
Article Rating
Inline Feedbacks
View all comments
12 years ago


12 years ago

hello P’aoddy

It is a good idea for set subnet but it can give faster than your old net [before you are sitting the router] ??

i will msn with you
see you ^^