IP Addresses and ARP

Document revision:0.9 (Fri Mar 05 08:35:08 GMT 2004)
Applies to: V2.8

General Information

Summary

The following Manual discusses IP address management and the Address Resolution Protocol settings. IP addresses serve as identification when communicating with other network devices using the TCP/IP protocol. In turn, communication between devices in one physical network proceeds with the help of Address Resolution Protocol and ARP addresses.

Specifications

Packages required: system
License required: Level1
Submenu level: /ip address, /ip arp
Standards and Technologies: IP, ARP
Hardware usage: Not significant

Related Documents

IP Addressing

Submenu level: /ip address

Description

IP addresses serve for a general host identification purposes in IP networks. Typical (IPv4) address consists of four octets. For proper addressing the router also needs the network mask value, id est which bits of the complete IP address refer to the address of the host, and which - to the address of the network. The network address value is calculated by binary AND operation from network mask and IP address values. It's also possible to specify IP address followed by slash "/" and amount of bits assigned to a network mask.

In most cases, it is enough to specify the address, the netmask, and the interface arguments. The network prefix and the broadcast address are calculated automatically.

It is possible to add multiple IP addresses to an interface or to leave the interface without any addresses assigned to it. Leaving a physical interface without an IP address is not a must when the bridging between interfaces is used (starting from RouterOS version 2.8). In case of bridging, the IP address can be assigned to any interface in the bridge, but actually the address will belong to the bridge interface. You can use /ip address print detail to see to which interface the address belongs to.

MikroTik RouterOS has following types of addresses:

Property Description

address (IP address) - IP address of the host

broadcast (IP address; default: 255.255.255.255) - broadcasting IP address, calculated by default from an IP address and a network mask

disabled (yes | no; default: no) - specifies whether the address is disabled or not

interface (name) - interface name the IP address is assigned to

actual-interface (read-only: name) - only applicable to logical interfaces like bridges or tunnels. Holds the name of the actual hardware interface the logical one is bound to.

netmask (IP address; default: 0.0.0.0) - specifies network address part of an IP address

network (IP address; default: 0.0.0.0) - IP address for the network. For point-to-point links it should be the address of the remote end

Notes

You cannot have two different IP addresses from the same network assigned to the router. Exempli gratia, the combination of IP address 10.0.0.1/24 on the ether1 interface and IP address 10.0.0.132/24 on the ether2 interface is invalid, because both addresses belong to the same network 10.0.0.0/24. Use addresses from different networks on different interfaces, or enable proxy-arp on ether1 or ether2.

Example

[admin@MikroTik] ip address> add address=10.10.10.1/24 interface=ether2
[admin@MikroTik] ip address> print
Flags: X - disabled, I - invalid, D - dynamic
  #   ADDRESS            NETWORK         BROADCAST       INTERFACE
  0   2.2.2.1/24         2.2.2.0         2.2.2.255       ether2
  1   10.5.7.244/24      10.5.7.0        10.5.7.255      ether1
  2   10.10.10.1/24      10.10.10.0      10.10.10.255    ether2

[admin@MikroTik] ip address>

Address Resolution Protocol

Submenu level: /ip arp

Description

Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. Address Resolution Protocol is used to map OSI level 3 IP addreses to OSI level 2 MAC addreses. A router has a table of currently used ARP entries. Normally the table is built dynamically, but to increase network security, it can be built statically by means of adding static entries.

Property Description

address (IP address) - IP address to be mapped

interface (name) - interface name the IP address is assigned to

mac-address (MAC address; default: 00:00:00:00:00:00) - MAC address to be mapped to

Notes

Maximal number of ARP entries is 1024.

If arp feature is turned off on the interface, i.e., arp=disabled is used, ARP requests from clients are not answered by the router. Therefore, static arp entry should be added to the clients as well. For example, the router's IP and MAC addresses should be added to the Windows workstations using the arp command:

C:\> arp -s 10.5.8.254  00-aa-00-62-c6-09

If arp property is set to reply-only on the interface, then router only replies to ARP requests. Neighbour MAC addresses will be resolved using /ip arp statically set table only

Example

[admin@MikroTik] ip arp> add address=10.10.10.10 interface=ether2 mac-address=06 \
\... :21:00:56:00:12
[admin@MikroTik] ip arp> print
Flags: X - disabled, I - invalid, H - DHCP, D - dynamic
  #   ADDRESS         MAC-ADDRESS       INTERFACE
  0 D 2.2.2.2         00:30:4F:1B:B3:D9 ether2
  1 D 10.5.7.242      00:A0:24:9D:52:A4 ether1
  2   10.10.10.10     06:21:00:56:00:12 ether2
[admin@MikroTik] ip arp>

If static arp entries are used for network security on an interface, you should set arp to 'reply-only' on that interface. Do it under the relevant /interface menu:

[admin@MikroTik] ip arp> /interface ethernet set ether2 arp=reply-only
[admin@MikroTik] ip arp> print
Flags: X - disabled, I - invalid, H - DHCP, D - dynamic
  #   ADDRESS         MAC-ADDRESS       INTERFACE
  0 D 10.5.7.242      00:A0:24:9D:52:A4 ether1
  1   10.10.10.10     06:21:00:56:00:12 ether2

[admin@MikroTik] ip arp>

Proxy-ARP feature

Description

All physical interfaces, like Ethernet, Atheros and Prism (wireless), Aironet (PC), WaveLAN, etc., can be set to use the Address Resolution Protocol or not. The other possible setting is to use Proxy-ARP feature. The Proxy-ARP means that the router will be listening to ARP requests on the relevant interface and respond to them with it's own MAC address, if the requests matches any other IP address of the router.

This can be usefull, for example, if you want to assign dial-in (ppp, pppoe, pptp) clients IP addresses from the same address space as used on the connected LAN.

Example

Consider the following configuration:

Proxy-ARP example

The MikroTik Router setup is as follows:

admin@MikroTik] ip arp> /interface ethernet print
Flags: X - disabled, R - running
  #    NAME                 MTU   MAC-ADDRESS       ARP
  0  R eth-LAN              1500  00:50:08:00:00:F5 proxy-arp
[admin@MikroTik] ip arp> /interface print
Flags: X - disabled, D - dynamic, R - running
  #    NAME                 TYPE             MTU
  0    eth-LAN              ether            1500
  1    prism1               prism            1500
  2 D  pppoe-in25           pppoe-in
  3 D  pppoe-in26           pppoe-in
[admin@MikroTik] ip arp> /ip address print
Flags: X - disabled, I - invalid, D - dynamic
  #   ADDRESS            NETWORK         BROADCAST       INTERFACE
  0   10.0.0.217/24      10.0.0.0        10.0.0.255      eth-LAN
  1 D 10.0.0.217/32      10.0.0.230      0.0.0.0         pppoe-in25
  2 D 10.0.0.217/32      10.0.0.231      0.0.0.0         pppoe-in26
[admin@MikroTik] ip arp> /ip route print
Flags: X - disabled, I - invalid, D - dynamic, J - rejected,
C - connect, S - static, R - rip, O - ospf, B - bgp
    #    DST-ADDRESS        G GATEWAY         DISTANCE INTERFACE
    0  S 0.0.0.0/0          r 10.0.0.1        1        eth-LAN
    1 DC 10.0.0.0/24        r 0.0.0.0         0        eth-LAN
    2 DC 10.0.0.230/32      r 0.0.0.0         0        pppoe-in25
    3 DC 10.0.0.231/32      r 0.0.0.0         0        pppoe-in26
[admin@MikroTik] ip arp>

Unnumbered Interfaces

Description

Unnumbered interfaces can be used on serial point-to-point links, e.g., MOXA or Cyclades interfaces. A private address should be put on the interface with the network being the same as the address on the router on the other side of the p2p link (there may be no IP on that interface, but there is an ip for that router).

Example

[admin@MikroTik] ip address> add address=10.0.0.214/32 network=192.168.0.1 \
\... interface=pppsync
[admin@MikroTik] ip address> print
Flags: X - disabled, I - invalid, D - dynamic
  #   ADDRESS            NETWORK         BROADCAST       INTERFACE
  0   10.0.0.214/32      192.168.0.1     192.168.0.1     pppsync
[admin@MikroTik] ip address>
[admin@MikroTik] ip address> .. route print detail
Flags: X - disabled, I - invalid, D - dynamic, J - rejected,
C - connect, S - static, R - rip, O - ospf, B - bgp
    0  S dst-address=0.0.0.0/0 preferred-source=0.0.0.0 gateway=192.168.0.1
         gateway-state=reachable distance=1 interface=pppsync

    1 DC dst-address=192.168.0.1/32 preferred-source=10.0.0.214
         gateway=0.0.0.0 gateway-state=reachable distance=0 interface=pppsync

[admin@MikroTik] ip address>

As you can see, a dynamic connected route has been automatically added to the routes list. If you want the default gateway be the other router of the p2p link, just add a static route for it. It is shown as 0 in the example above.

© Copyright 1999-2006, MikroTik. All rights reserved. Mikrotik, RouterOS and RouterBOARD are trademarks of Mikrotikls SIA. Other trademarks and registered trademarks mentioned herein are properties of their respective owners.