Actually it is a windows 7 problem, try below steps to solve it:
You may try to disable the DHCP BROADCAST flag in Windows 7 computers & check if the issue persists. To do this, follow these steps.
Important: This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
** (http://support.microsoft.com/kb/322756/ ) How to back up and restore the registry in Windows. **
A. Click Start, type ‘regedit’ in the Start Search box, and then click ‘regedit’ in the Programs list.
Note: If you are prompted for an administrator password or for confirmation, type your password, or click Continue.
B. Locate and then click the following registry subkey:
C. In this registry path, click the (GUID) subkey that corresponds to the network adapter that is connected to the network.
D. On the Edit menu, point to New, and then click DWORD (32-bit) Value.
E. In the New Value #1 box, type ‘DhcpConnEnableBcastFlagToggle’, and then press ENTER.
F. Right-click’ DhcpConnEnableBcastFlagToggle’, and then click Modify.
G. In the Value data box, type 1, and then click OK.
H. Close Registry Editor.
By setting this registry key to 1, Windows 7 will first try to obtain an IP address by using the BROADCAST flag in DHCP Discover packets. If that fails, it will try to obtain an IP address without using the BROADCAST flag in DHCP Discover packets.
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